The Mexican-American War was fought between the United States and Mexico between 1846 and 1848. It is also called the U.S.-Mexico War. In the U.S. it is also known as the Mexican War; in Mexico it is known as the U.S. Intervention, the U.S. Invasion of Mexico, the United States War Against Mexico, and the War of Northern Aggression (this last name is more commonly used in the American South to refer to the American Civil War).
The war grew out of unresolved conflicts between Mexico and Texas. After winning its independence from Mexico in 1836, the Republic of Texas was annexed by the United States in 1845; however, the southern and western borders of Texas remained disputed during the Republic's lifetime. That same year tensions between the two countries over territory were raised when the United States government offered to pay off the Mexican debt to American settlers if Mexico allowed the U.S. to purchase the territories of Alta California and Nuevo México from Mexico.
The U.S. government claimed that the southern border of Texas was the Rio Grande; Mexico maintained it to be the Nueces River. President James K. Polk ordered General Zachary Taylor to place troops between the two rivers. Taylor crossed the Nueces, ignoring Mexican demands that he withdraw, and marched south to the Rio Grande where he began to build Fort Brown.
Fighting began on April 24, 1846 when Mexican cavalry captured one of the American detachments near the Rio Grande. After the border clash and battles at Palo Alto and Resaca de la Palma, Polk requested a declaration of war, announcing to Congress that the Mexicans had "invaded our territory and shed American blood upon American soil". The U.S. Congress declared war on May 13, 1846. Northerners and Whigs generally opposed the war while Southerners and Democrats tended to support it. Mexico declared war on May 23.
After the declaration of war, U.S. forces invaded Mexican territory on several fronts. In the Pacific, the U.S. Navy sent John D. Sloat to occupy California and claim it for the U.S. because of concerns that Britain might also attempt to occupy the area. He linked up with Anglo colonists in Northern California who had previously declared an independent California Republic and occupied some key cities. Meanwhile, U.S. army troops under Stephen W. Kearny occupied Santa Fe, New Mexico, and Kearny led a small force to California where, after some initial reverses, he united with naval reinforcements under Robert F. Stockton to occupy San Diego and Los Angeles.
The main force led by Taylor continued across the Rio Grande into Mexico, winning the Battle of Monterrey in September 1846. Mexican President Antonio López de Santa Anna personally marched north to fight Taylor but was defeated at the battle of Buena Vista on February 22, 1847. Meanwhile, rather than reinforce Taylor's army for a continued advance, President Polk sent a second army under U.S. general Winfield Scott in March, which was transported to the port of Veracruz by sea, to begin an invasion of the Mexican heartland. Scott won the Siege of Veracruz and marched toward Mexico City, winning the battles of Cerro Gordo and Chapultepec and occupying the Mexican capital.
The Treaty of Cahuenga, signed on January 13, 1847, ended the fighting in California. The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, ended the war and gave the USA undisputed control of Texas as well as California, Nevada, Utah, and parts of Colorado, Arizona, New Mexico, and Wyoming. In return, Mexico received $18,250,000 or the equivalent of $627,482,629 in today's dollars, total for the cost of the war.
During the course of the war, around 13,000 American soldiers were killed. Of these deaths, only about 1.5% (~195) were from actual combat; the rest stemmed from disease and unsanitary conditions during the war. It is also estimated that, if post-war deaths from war-related causes are counted, the combined U.S. casualty rate for the war was very high, 30-40%. Mexican casualties remain somewhat of a mystery, and are estimated at 25,000.
During the War, many important battles were fought, including the Battle of Churubusco and the Battle of Padierna. One of the most important actors in the Mexican side was Pedro María Anaya, who defended the convent of Santa María Churubusco. He said the most famous phrase of the War when general Twiggs entered into the convent: "If we had bullets you weren't here". It shows how the advantages of the American army were not only organisation, but also wealth.
A noteworthy, if controversially–remembered, group of fighters was Saint Patrick's Battalion (San Patricios), a group of several hundred immigrant soldiers (mostly from Ireland) who deserted the U.S. Army in favor of the Mexican side. According to one version of events, the Battalion deserted after having experienced harsh religious discrimination in the United States, and found common cause with Mexico due to its status as a largely Catholic country. Most would die in the Battle of Churubusco. Some were captured and hanged, reputedly by generals instructed to make sure that the last thing they saw was the lowering of the Mexican flag and the raising of the U.S. flag. Some historians claim that these men were actually prisoners of war and forced to fight for Mexico. Others argue that they were simply traitors and deserters, and in some cases were marked with a "D" (of deserters) with hot iron. It is known that some of the captured Irish soldiers were choked in Mixcoac. There are, in any event, a number of monuments to these soldiers in present-day Mexico.
Mexico lost much of its territory in the war, leaving it with a lasting bitterness towards the United States. Santa Anna fled to exile in Venezuela. General Porfirio Díaz, President of Mexico from 1877–1911, would later lament: "¡Pobre México! Tan lejos de Dios, y tan cerca de los Estados Unidos." ("Poor Mexico! So far from God, and so close to the United States.")
In the United States, victory in the war brought a surge in patriotism as the acquisition of new western lands – the country had also acquired the southern half of the Oregon Country in 1846 – seemed to fulfill citizens' belief in their country's Manifest Destiny. While 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." The war made a national hero of Zachary Taylor, a Southern Whig, who was elected president in the election of 1848.
However, this period of national euphoria would not last long. The war had been widely supported in the southern states but largely opposed in the northern states. This division largely developed from expectations of how the expansion of the United States would affect the issue of slavery. At the time, Texas recognized the institution of slavery, but Mexico did not. Many Northern abolitionists viewed the war as an attempt by the slave-owners to expand slavery and assure their continued influence in the federal government. Henry David Thoreau wrote his essay Civil Disobedience and refused to pay taxes because of this war.
The main issue which furthered sectionalism was the expansion of slavery into the national territories. The Missouri Compromise of 1820 banned slavery in national territories north of 36 degrees, 30 minutes (roughly the southern border of Missouri, although that state had been exempted). Also, the Senate was constructed to give equal balance to slave and free states. The Missouri Compromise, however, left room for more free states than slave states and, if continued, would upset the balance of power within the Senate. Thus, many Southerners supported the war to provide more room for slavery to expand (believing that if slavery were not allowed to continue to expand, it would ultimately die out). There were proposals during this time to split Texas (which was easily the largest state in the Union geographically) into multiple slave states, but this did not come to pass.
During the first year of the war, Congressman David Wilmot introduced a bill which would prohibit slavery in any new territory captured from Mexico. This bill, which became known as the Wilmot Proviso caused an immediate outcry from Southerners on both sides of the congressional aisle. To Southerners, it looked as if the north was willing to abandon parity within the senate, and the Wilmot Proviso sparked further hostility between the sections. The bill itself was passed by the House of Representatives but failed in the Senate, with both votes on sectional lines.
In 1848 Democrats proposed a new solution to the issue of whether territories should have slavery, known as popular sovereignty. This would allow for voters within a territory to determine for themselves whether or not they would allow slavery within their territory. The Kansas-Nebraska Act of 1854 would make popular sovereignty the law of the land, striking down the Missouri Compromise. In protest of this, the Republican Party was organized that year by opponents of the expansion of slavery.
Taylor, the hero of Monterrey and Buena Vista, was elected President in the canvass of November, 1848 running as a Whig but without a political history. He would not have the opportunity to resolve the intersectional problems stemming from the war, as his death in 1850 resulted in the elevation of Whig party insider Millard Fillmore to the Presidency. Fillmore turned to the problem of resolving the Texas boundary and territorial slavery disputes arising from the war, enlisting the aid of Whig party stalwarts Henry Clay and Daniel Webster. The three were able to fashion the Compromise of 1850 in an effort to produce a final solution, but this compromise created more disputes, as well as helping to end the Whig party as a national force. The Whigs made one more effort to function as a national party, nominating the other key war general Winfield Scott after a bitter party convention that split its vote between Scott, Fillmore, and Webster. Surprisingly, Scott's campaign came apart not so much due to disagreements over the compromise and slavery, but rather the newly vexacious anti-immigrant and anti-Catholic sentiments which were spilling over into political discourse.
Ulysses S. Grant, who served in the war under Scott's command, would later consider the war to be one of the causes of the American Civil War: "The occupation, separation and annexation [of Texas] were ... a conspiracy to acquire territory out of which slave states might be formed for the American Union." Many of the generals of the latter war had fought in the former, including Grant, Ambrose Burnside, Stonewall Jackson, George Meade, and Robert E. Lee, as well as future President of the Confederacy Jefferson Davis.