What most Americans are on Denial Stage. The Invasion of United States to Mexico.
Before this War was engage by both Countries U.S. was already raising their flag on California, Texas, Arizona, Oregon and New Mexico. Do you believe this war was a fair practice of War or just was an another Ironic Conspiracy and unjustified Invasion by U.S.?
Compare Polk's actions with that of Iraq during the Persian Gulf War. Saddam Hussein justified his invasion of Kuwait based upon old territorial claims.

The Mexican–American War, also known in the United States as the Mexican War, and in Mexico as La Invasión Estadounidense (the United States Invasion), La Intervención Norteamericana (the North American Intervention), La Guerra de Defensa (the war of defence), or La Guerra del 47 (the War of '47) was a military conflict fought between the United States and Mexico from 1846 to 1848, in the wake of the 1845 U.S. annexation of Texas. Mexico had not recognized the secession of Texas in 1836 and announced its intention to take back what it considered a rebel province.

In the United States, the war was a partisan issue, supported by most Democrats and opposed by most Whigs, with popular belief in the Manifest Destiny of the United States ultimately translating into public support for the war. In Mexico, the war was considered a matter of national pride.

The most important consequence of the war was the Mexican Cession, in which the Mexican territories of Alta California and Santa Fé de Nuevo México,were ceded to the United States under the terms of the Treaty of Guadalupe Hidalgo. In Mexico, the enormous loss (52%) of territory encouraged the central government to enact policies to colonize its northern territories as a hedge against further losses.
After Mexico gained independence from the Spanish Empire at the end of the Mexican War of Independence in 1821, the Mexican Empire inherited ownership of the provinces of Alta California, Nuevo Mexico, and Texas, from Spain. (These territories are now States within the United States of America.) Weakened and virtually bankrupt from the Mexican War of Independence, the new Mexican government found it difficult to govern its northern territories, which in any case were thousands of miles from Mexico City, the capital.
Although the United States made overtures to the Mexican government to buy Texas, Mexico, ruled by Agustín I of Mexico (Iturbide), opposed it. Mexico intended to colonize its northern provinces.
Mexico's Constitution of 1835 prohibited slavery. Many Texans were slave owners who had emigrated from Tennessee and other slave states in the United States. These Texans objected to the abolition of slavery. They were further incensed in 1836 when General Antonio López de Santa Anna abolished the 1835 constitution and established a new constitution that attempted to centralize power. The new centralist constitution enshrined the Siete Leyes, which included secular reforms but granted additional powers to the president, such as the power to close congress and suppress the judiciary. Several Mexican states rebelled against the new central government under Santa Anna, including Texas (then a department of the state of Coahuila y Tejas), San Luis Potosí, Querétaro, Durango, Guanajuato, Michoacán, Yucatán, Jalisco and Zacatecas.
A violent insurgency, known as the Texas Revolution, started in 1836. General Santa Anna responded by engaging in two battles against rebel forces, the Álamo and Goliad, which encouraged a wider revolt in Texas.

Republic of Texas

The Republic of Texas.The present-day outlines of the U.S. states are superimposed on the boundaries of 1836–1845.In the 1836 Texas Revolution, Texas won its independence after defeating Santa Anna and the Mexican army. General Santa Anna was taken captive by the Texan's militia and released only after signing the Treaties of Velasco in which he promised to recognize the sovereignty of the Republic of Texas and the Rio Grande as the Texian-Mexican boundary. When he returned to Mexico City the government refused to recognize the loss or independence of the Republic of Texas because, it said, Santa Anna was not a representative of Mexico and that he signed away Texas under duress. Mexico declared its intention to recapture what it considered a breakaway state.
In the decade after 1836, Texas consolidated its position as an independent republic by establishing diplomatic ties with Britain, France, and the United States. Most Texans were in favor of annexation by the United States, but U.S. President Andrew Jackson rejected it.

Under U.S. President James K. Polk, Texas was admitted to the union in 1845, when it became the 28th state. The Mexican government had long warned that annexation meant war with the United States. Britain and France, which recognized the independence of Texas, repeatedly tried to dissuade Mexico from declaring war against a much more powerful neighbor. British efforts to mediate were fruitless in part because additional political disputes (particularly the Oregon boundary dispute) arose between Mexico, Britain and the United States.
In 1845 U.S. President James K. Polk sent diplomat John Slidell to Mexico City in an attempt to purchase Mexico's California and New Mexico territories. U.S. expansionists wanted California as a way to thwart British ambitions in the area and to have a Pacific Ocean port. Polk authorized Slidell to forgive the $4.5 million owed to U.S. citizens for damages caused by the Mexican War of Independence and pay another $25 to $30 million in exchange for the two territories.
However, Mexico was not inclined nor in a position to negotiate. In 1846 alone, the presidency changed hands four times, the war ministry six times, and the finance ministry sixteen times.[1] Mexican public opinion and Mexican political factions and leaders felt Mexico's honor would be diminished by selling any territory.[2] Mexicans opposing open conflict with the United States, including President José Joaquín de Herrera, were viewed as traitors. When de Herrera considered receiving Slidell in order to peacefully negotiate the problem of Texas annexation, he was accused of treason and deposed.
Military opponents of President José Joaquín de Herrera supported by populous newspapers, considered Slidell's presence in Mexico City an insult. After a more nationalistic government under General Mariano Paredes y Arrillaga came to power, the new government publicly reaffirmed Mexico's claim to Texas, and Slidell left convinced that Mexico should be "chastized." [2]

Opening hostilities
Mexico, which had refused to recognized Texas's independence, claimed the Nueces River — about 150 miles (240 km) north of the Rio Grande — as the border between Texas and Mexico. The United States, however, upheld Texas' claim to the land between the Nueces and the Rio Grande. The boundary claimed by Texas was established by the 1836 Treaty of Velasco that ended the Texas Revolution. Mexico argued that General Santa Anna signed the treaty under duress when he was held captive by the Texans. Moreover, the Mexicans argued, Santa Anna had no authority to negotiate or sign a treaty, and the treaties were never ratified by the Mexican government. In 1846, after Texas was admitted into the Union, Polk sent militia under General Zachary Taylor to the Rio Grande to protect Texas.
Taylor ignored Mexican demands to withdraw to the Nueces and began constructing a make-shift fort (later known as Fort Brown) on the banks of the Rio Grande opposite the Mexican town of Matamoros. Mexican forces under General Mariano Arista prepared for war.
On April 24, 1846, a 2,000-strong Mexican cavalry detachment attacked a 63-man U.S. patrol that was sent into the contested territory north of the Rio Grande and south of the Nueces River. The Mexican cavalry succeeded in routing the patrol, killing 11 U.S. soldiers in what later became known as the Thornton Affair after the slain U.S. officer who was in command. A few survivors escaped and returned to Fort Brown.

On May 3, Mexican artillery at Matamoros opened fire on Fort Brown, which replied with its own guns. The bombardment continued for five days and expanded as Mexican forces gradually surrounded the fort. Two U.S. soldiers were killed during the bombardment, including Jacob Brown, after whom the fort was later named.

On May 8, Zachary Taylor arrived with 2,400 troops to relieve the fort. However, Arista rushed north and intercepted him with a force of 3,400 at Palo Alto. The Americans used a new artillery method named flying artillery — a mobile light artillery that was mounted on horse carriages, with all cannoneers mounted as well. U.S. artillery had a devastating effect on the Mexican Army. The Mexicans responded with cavalry skirmishes and its own artillery. The U.S. flying artillery somewhat demoralized the Mexican side, and they felt the need to find a terrain more to their advantage. They retreated to the far side of a dry riverbed (resaca) during the night, which provided a natural fortification, but they also scattered their troops so that communication was difficult. During the Battle of Resaca de la Palma the next day, the two sides engaged in vicious hand-to-hand fighting. The U.S. cavalry managed to capture the Mexican artillery, leading the Mexican side to retreat—a retreat that turned into a rout. Because of the terrain and the dispersion of his troops, Arista found it impossible to rally his forces. Mexican casualties were heavy, and the Mexicans were forced to abandon their artillery and baggage. Fort Brown inflicted further casualties as the withdrawing troops passed by them and swam across the Rio Grande where many drowned.

Declaration of war
By then, Polk had received word of the Thornton Affair. This, added to the Mexican government's rejection of Slidell, Polk believed, constituted a casus belli. His message to Congress on May 11, 1846 stated that Mexico had "invaded our territory and shed American blood upon the American soil." A joint session of Congress approved the declaration of war, with southern Democrats in strong support because they saw the annexation of Mexico as an opportunity to increase the number of slave states. Sixty-seven Whigs voted against the war on a key slavery amendment, but on the final passage only 14 Whigs voted no, including Reps. Abraham Lincoln and John Quincy Adams. The United States declared war on Mexico on May 13, 1846. Mexico officially declared war on July 7 (sometimes the manifest from President Paredes on May 23 is considered a declaration of war, but only the Mexican congress had the power to declare war).

Opposition to the war
In the United States, most Whigs in the North and South opposed the war; most Democrats supported it. Joshua Giddings led a group of dissenters in Washington D.C. He called the war with Mexico "an aggressive, unholy, and unjust war," and voted against suppling soldiers and weapons for the war. He said:

"In the murder of Mexicans upon their own soil, or in robbing them of their country, I can take no part either now or here-after. The guilt of these crimes must rest on others. I will not participate in them."
Fellow Whig, Congressman Abraham Lincoln, contested the causes for the war and demanded to know the exact spot on which Thornton had been attacked and U.S. blood shed. "Show me the spot," he demanded. Whig leader Robert Toombs of Georgia declared:

"This war is a nondescript.... We charge the President with usurping the war-making power... with seizing a country... which had been for centuries, and was then in the possession of the Mexicans.... Let us put a check upon this lust of dominion. We had territory enough, Heaven knew."[3]

Northern abolitionists attacked the war as an attempt by slave-owners -- frequently referred to as "the Slave Power" -- to expand the grip of slavery and thus assure their continued influence in the federal government. Acting on his convictions, Henry David Thoreau was jailed for his refusal to pay taxes to support the war, and penned his famous essay, Civil Disobedience.
Former President John Quincy Adams also expressed his belief that the war was fundamentally an effort to expand slavery. In response to such concerns, Democratic Congressman David Wilmot introduced the Wilmot Proviso, which aimed to prohibit slavery in any new territory acquired from Mexico. Wilmot's proposal did not pass Congress, but it spurred further hostility between the factions.
Conduct of the war
After the declaration of war, U.S. forces invaded Mexican territory on two main fronts. The U.S. war department sent a cavalry force under Stephen W. Kearny to invade western Mexico from Fort Leavenworth, reinforced by a Pacific fleet under John D. Sloat. This was done primarily because of concerns that Britain might also attempt to occupy the area. Two more forces, one under John E. Wool and the other under Taylor, were ordered to occupy Mexico as far south as the city of Monterrey.
Letter from a Naval surgeon on board the frigate "Potomac" en route to Mexico

A young Navy assistant surgeon, Oscar F. Baxter, a native of North Carolina, when on his way to Vera Cruz during the Mexican War, wrote a letter to his fiancée. The letter was dated October 1, 1846 and was posted from Pensacola, Florida when his ship the frigate "Potomac" made a stop at the Navy yard there. In it he reported that there had been a lot of scurvy among the men on the ship, but the worst seemed to be over. But another disease was causing him great stress: yellow fever, which had afflicted some on the ship and about forty more workers at the Pensacola Navy yard (including the surgeon there, for whom Dr. Baxter had to fill in).

He goes on to say that he has heard a "rumor" that the overtures of peace offered by our government have come. He was happy to report that he anticipated, in several months' time, to return to the United States to take an examination, which if passed, would promote him from assistant surgeon to surgeon; such promotion would raise his annual salary from $1,028 to $1,273.
He also reports:

The blockading squadron off Vera Cruz has recently captured several rich prizes--the value of the two last is estimated at $150,000. They will be brought in and be judged by a Court of Admiralty whether they are legal prizes or not.
After all of that, he includes the following in his letter:

A man by the name of Samuel Jackson, seaman on board of the St. Mary's (a sister ship to the "Potomac" in the squadron en route to Mexico) has been sentenced to death by a Court Martial recently held upon the charges preferred against him for mutiny. The Commodore has approved the sentence and the day has been appointed for carrying it into effect. An example of the kind becomes sometimes an act of humanity. If he be let off, a dozen may ultimately may have to suffer his sentence carried into effect. It is possible the Commodore may reprieve him. If he does it will be a death blow to the discipline of the Squadron.
Actually, Jackson had already been executed on September 21, 1846, according to the account published in The Daybook included below. Evidently, Dr. Baxter had heard of the incident on his way to Pensacola, but had not yet learned of the execution when he wrote his letter on October 1st.

(The actual letter referred to above is in the possession of Dr. Baxter's great-grandson, who lives in Baltimore County, Maryland. Later, after North Carolina seceded from the Union at the time of the Civil War, Dr. Baxter became a surgeon in the Confederate army. Excerpts from several letters to his twelve-year-old daughter in 1864 from the field of battle at Chaffin's Bluff in Virginia (at which time he was a widower) describe the intense fighting going on around his field hospital and the large number of badly-wounded men, both Confederate and Union, that he had to treat.)

Several weeks into the (Mexican) war, discipline continued to be a problem...throughout the squadron. Despite weekly floggings, officers and sailors continue to violate basic rules of discipline...Tired of the chaos, (the squadron commander) decided to make an example of one sailor. The unlucky fellow was Seaman Samuel Jackson of St. Mary’s. Jackson had assaulted his division officer. Though everyone in the squadron, including the lieutenant who was attacked, thought it to be excessive, an execution went forward. Rev. Taylor made many trips over to St. Mary’s to see the condemned man and to convert him to Christianity before his death...on the morning of September 21, Jackson was hung from the yardarm in front of the entire squadron. While many disagreed with (the squadron commander's) decision, there were far fewer discipline problems for the rest of the war.

California
When the US declared war against Mexico, on May 13, 1846, it took almost two months (mid-July 1846) for definite word of war to get to California. U.S. consul Thomas O. Larkin, stationed in Monterey, on hearing rumors of war tried to keep peace between the U.S. and the small Mexican military garrison commanded by José Castro. U.S. Army captain John C. Frémont with about 60 well-armed men had entered California in December 1845 and was making a slow march to Oregon when they received word that war between Mexico and the U.S. was imminent. [4]
On June 15, 1846, some 30 settlers, mostly U.S. citizens, staged a revolt and seized the small Mexican garrison in Sonoma. They raised the "Bear Flag" of the California Republic over Sonoma. It lasted one week until the U.S. Army, led by Fremont, took over on June 23. The California state flag today is based on this original Bear Flag, and still contains the words "California Republic."
Commodore John Drake Sloat, on hearing of imminent war and the revolt in Sonoma, ordered his naval forces to occupy Yerba Buena (present San Francisco) on July 7 and raise the American flag. When Mexico did not ceclare the war yet. they were Raising the American Flag. On July 15, Sloat transferred his command to Commodore Robert F. Stockton, a much more aggressive leader, who put Frémont's forces under his orders. On July 19, Frémont's "California Battalion" swelled to about 160 additional men from newly arrived settlers near Sacramento, and he entered Monterey in a joint operation with some of Stockton's sailors and marines. The word had been received — the war was official. The U.S. forces easily took over the north of California; within days they controlled San Francisco, Sonoma, and Sutter's Fort in Sacramento.

In Northern California, Mexican General José Castro and Governor Pío Pico fled to Mexico . When Stockton's forces, sailing south to San Diego, stopped in San Pedro, he dispatched 50 US Marines, and entered Los Angeles unresisted on August 13, 1846, known as the Siege of Los Angeles, the nearly bloodless conquest of California seemed complete. Stockton, however, left too small a force in Los Angeles, and the Californios, acting on their own and without help from Mexico, led by José Mariá Flores , forced the American garrison to retreat in late September. More than 300 reinforcements sent by Stockton, led by U.S. Navy Captain William Mervine, were repulsed in the Battle of Dominguez Rancho, October 7 through October 9, 1846, near San Pedro, where 14 U.S. Marines were killed. The rancho vaqueros, banded together to defend their land, fighting as Californio Lancers, became a force to deal with the Americans had not planned on. Meanwhile, General Stephen W. Kearny, with a squadron of 139 dragoons, finally reached California after a grueling march across New Mexico, Arizona and the Sonora desert, on December 6, 1846, and was defeated by the Californio Lancers at the Battle of San Pasqual near San Diego, California, where 22 of Kearny's troops were killed.
Stockton rescued Kearny's retreating forces and later, with their re-supplied, combined force, marched north from San Diego, entering the Los Angeles area on January 8, 1847, linking up with Frémont's men. With U.S. forces totaling 660 soldiers and marines, they fought and defeated the 160 man Californio force in the decisive Battle of Rio San Gabriel, and the next day, January 9, 1847, they fought the Battle of La Mesa. On January 12, 1847, the last significant body of Californios surrendered to U.S. forces. That marked the end of the War in California. On January 13, 1847, the Treaty of Cahuenga was signed.

On January 28, 1847, U.S. Army Lieutenant William Tecumseh Sherman and his army unit arrived in Monterey, California as U.S. forces in the pipeline continued to stream into California. On March 15, 1847, Col. Jonathan D. Stevenson’s Seventh Regiment of New York Volunteers of about 900 men started arriving in California. All of these men were in place when word went out that gold was discovered in California, January 1848.

Northeastern Mexico
The defeats at Palo Alto and Resaca de la Palma caused political turmoil in Mexico, turmoil which Antonio López de Santa Anna used to revive his political career and return from self-imposed exile in Cuba. He promised the U.S. troops that if allowed to pass through their blockade, he would negotiate a peaceful conclusion to the war and sell the New Mexico and California territories to the United States. Once he arrived in Mexico, however, he reneged and offered his military skills to the Mexican government. After he had been appointed general he reneged again and seized the presidency.

2,300 U.S. troops led by Taylor crossed the Rio Grande (Rio Bravo) after some initial difficulties in obtaining river transport. He occupied the city of Matamoros, then Camargo (where while waiting the soldiery suffered the first of many problems with disease) and then proceeded south and besieged the city of Monterrey. This Battle of Monterrey was a hard fought battle during which both sides suffered serious losses. The American light artillery was ineffective against the stone fortifications of the city. The Mexican forces were under General Pedro de Ampudia. A U.S. infantry division and the Texas Rangers captured four hills to the west of the town and with them heavy cannon. That lent the U.S. soldiers the strength to storm the city from the west and east. Once in the city, U.S. soldiers fought house to house: each was cleared by throwing lighted shells, which worked like grenades. Eventually, these actions drove and trapped Ampudia's men into the city's central plaza, where howitzer shelling forced Ampudia to negotiate. Taylor agreed to allow the Mexican Army to evacuate and to an 8-week armistice in return for the surrender of the city. Under pressure from Washington, Taylor broke the armistice and occupied the city of Saltillo, southwest of Monterrey. Santa Anna blamed the loss of Monterrey and Saltillo on Ampudia and demoted him to command a small artillery battalion.


Scott's campaign
Rather than reinforce Taylor's army for a continued advance, President Polk sent a second army under General Winfield Scott, which was transported to the port of Veracruz by sea, to begin an invasion of the Mexican heartland. Scott performed the first major amphibious landing in the history of the United States in preparation for the Siege of Veracruz. A group of 12,000 volunteer and regular soldiers successfully offloaded supplies, weapons and horses near the walled city. Included in the invading force were Robert E. Lee, George Meade, Ulysses S. Grant, and Thomas "Stonewall" Jackson.
The city was defended by Mexican General Juan Morales with 3,400 men. Mortars and naval guns under Commodore Matthew C. Perry were used to reduce the city walls and harass defenders. The city replied as best it could with its own artillery. The effect of the extended barrage destroyed the will of the Mexican side to fight against a numerically superior force, and they surrendered the city after 12 days under siege. U.S. troops suffered 80 casualties, while the Mexican side had around 180 killed and wounded, about half of whom were civilian. During the siege, the U.S. side began to fall victim to yellow fever.

In May, Scott pushed on to Puebla, the second largest city in Mexico. Because of the citizens' hostility to Santa Anna, the city capitulated without resistance on May 1. Mexico City was laid open in the Battle of Chapultepec and subsequently occupied.

Winfield Scott became an American national hero after his victories in the Mexican War, and later became military governor of occupied Mexico City.

Treaty of Guadalupe Hidalgo

The Treaty of Guadalupe Hidalgo, signed on February 2, 1848 by American diplomat Nicholas Trist, ended the war and gave the U.S undisputed control of Texas, established the U.S.-Mexican border of the Rio Grande River, and ceded to the United States California, Nevada, Utah, and parts of Colorado, Arizona, New Mexico, and Wyoming. In return, Mexico received US $15,000,000, half the amount of money that the U.S. had attempted to offer in return for New Mexico and California alone three years earlier. This led a leading U.S. newpaper, the Whig intelligencer to conclude sardonically that;

“ We take nothing by conquest.... Thank God.[5][6] ”

The exchange of land is known as the Mexican Cession. Mexicans living in the conquered lands could choose to return to Mexico or stay and become American citizens. Article X was stricken from the treaty before it was ratified by the U.S. Senate. These articles promised that the United States would recognize Mexican and Spanish land grants.
In 1853, in what became known as The Gadsden Purchase, the United States paid an additional $10 million to Mexico to purchase land in what is now southern Arizona and southern New Mexico for the construction of a southern route for a transcontinental railroad. The purchase was also designed to further compensate Mexico for the lands taken by the U.S. after the Mexican-American War.

ResultsMexico lost more than 500,000 square miles (about 1,300,000 square km) of land, almost half of its territory. The annexed territories contained about 1,000 Mexican families in California and 7,000 in New Mexico. A few moved back to Mexico; the great majority remained in the US. Descendants of these Mexican families have risen to prominence in American life, such as United States Senator Ken Salazar, and his brother, U.S. Rep. John Salazar, both from Colorado.

A month before the end of the war, Polk was criticized in a United States House of Representatives amendment to a bill praising Major General Zachary Taylor for "a war unnecessarily and unconstitutionally begun by the President of the United States."
This criticism, in which Congressman Abraham Lincoln played an important role, followed congressional scrutiny of the war's beginnings, including factual challenges to claims made by President Polk[7] [8]. The vote followed party lines, with all Whigs supporting the amendment. Lincoln's attack haunted his future campaigns in the heavily Democratic state of Illinois, and was cited by enemies well into his presidency.[9] The stand did not cost Lincoln his Congressional seat in Illinois' Seventh Congressional District; the district was the only place in Illinois where a Whig could win high office, and party leaders agreed to one-term limits for Whig representatives there. Lincoln was succeeded by a Democrat, but the Seventh Congressional District voted for Zachary Taylor, a Whig, that fall.[10]

In much of the United States, victory and the acquisition of new land brought a surge of patriotism (the country had also acquired the southern half of the Oregon Country in 1846 through a treaty with Great Britain).
Victory seemed to fulfill citizens' belief in their country's Manifest Destiny. While Whig Ralph Waldo Emerson rejected war "as a means of achieving America's destiny," he accepted that "most of the great results of history are brought about by discreditable means." Although the Whigs had opposed the war, they made Zachary Taylor their presidential candidate in the election of 1848, praising his military performance while muting their criticism of the war itself.

In the 1880s, Ulysses S. Grant, who had served under Taylor's command, called the conflict an evil war that had brought God's punishment on the United States in the form of the American Civil War:

The Southern rebellion was largely the outgrowth of the Mexican war. Nations, like individuals, are punished for their transgressions. We got our punishment in the most sanguinary and expensive war of modern times. [3]
In the "Personal Memoirs of Ulysses S. Grant" published in 1885, Grant recalled his actions in the Mexican War with regret:

Generally, the officers of the army were indifferent whether the annexation was consummated or not; but not so all of them. For myself, I was bitterly opposed to the measure, and to this day regard the war, which resulted, as one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory. [4]

Many of the generals of the latter war had fought in the former, including Grant, George B. McClellan, Ambrose Burnside, Stonewall Jackson, James Longstreet, George Meade, and Robert E. Lee, as well as the future Confederate President Jefferson Davis. In Mexico City's Chapultepec Park, the Monument to the Heroic Cadets commemorates the heroic sacrifice of six teenaged military cadets who fought to their deaths rather than surrender to American invaders during the Battle of Chapultepec Castle on September 18, 1847. The monument is an important patriotic site in Mexico. On March 5, 1947, nearly one hundred years after the battle, U.S. President Harry S. Truman placed a wreath at the monument and stood for a moment of silence.

Combatants

On the American side, the war was fought by regulars and the volunteers, with the majority of atrocities being committed by volunteers. U.S. soldiers' memoirs describe cases of scalping innocent civilians, the rape and murder of women, the murder of children, the burning of homes, and the desecrating of Catholic religious objects and buildings. One officer's diary records:

"We reached Burrita about 5 pm, many of the Louisiana volunteers were there, a lawless drunken rabble. They had driven away the inhabitants, taken possession of their houses, and were emulating each other in making beasts of themselves.[11]

John L. O'Sullivan, a vocal proponent of Manifest Destiny, later recollected:

"The regulars regarded the volunteers with importance and contempt... [The volunteers] robbed Mexicans of their cattle and corn, stole their fences for firewood, got drunk, and killed several inoffensive inhabitants of the town in the streets."
Many of the volunteers were unwanted and considered poor soldiers. The expression "Just like Gaines's army" came to refer to something useless, the phrase having originated when a group of untrained and unwilling Louisiana troops were rejected and sent back by General Taylor at the beginning of the war.

One of the contributing factors to loss of the war by Mexico was the inferiority of their weapons. The Mexican army was using British muskets from the Napoleonic Wars; furthermore, Mexican troops were trained to fire with their muskets held loosely at hip-level[citation needed], while U.S. soldiers used the more accurate method of butting the rifle up to the shoulder and taking aim along the barrel. In contrast to the aging Mexican standard-issue infantry weapon, some U.S. troops had the latest U.S.-manufactured breech-loading flintlock "Hall's" rifles and Percussion cap Model 1841 rifles. In the later stages of the war, U.S. cavalry and officers were issued Colt revolvers, of which the U.S. army had ordered 1000 in 1846. Throughout the war, the superiority of the U.S. artillary often carried the day.

The Saint Patrick's Battalion (San Patricios) was a group of several hundred immigrant soldiers, the majority Irish, who deserted the U.S. Army because of ill-treatment or sympathetic leanings to fellow Mexican Catholics. They joined the Mexican army. Most were killed in the Battle of Churubusco; about 100 were captured by the U.S. and roughly half were hanged as deserters.

During the war political quarrels arose regarding the disposition of conquered Mexico. A strong "All-Mexico" movement urged annexation of the entire territory. Abolitionists opposed that position and fought for the exclusion of slavery from any territory absorbed by the United States. In 1847 the House of Representatives passed the Wilmot Proviso, stipulating that none of the territory acquired should be open to slavery. The Senate avoided the issue, and a late attempt to add it to the Treaty of Guadalupe Hidalgo was defeated.

The Treaty of Guadalupe Hidalgo was the result of Nicholas Trist's unauthorized negotiations. It was approved by the U.S. Senate on Mar. 10, 1848, and ratified by the Mexican Congress on May 25. Mexico's cession of California and New Mexico and its recognition of U.S. sovereignty over all of Texas north of the Rio Grande formalized the addition of 3.1 million km2 (1.2 million mi2) of territory to the United States. In return the United States agreed to pay $15 million and assumed the claims of its citizens against Mexico. A final territorial adjustment between Mexico and the United States was made by the Gadsden Purchase in 1853.