U.S. annexation of Texas sparked the war. The Texas Revolution of 1836 had won independence for the republic of Texas, but Mexico never officially recognized the loss of the province. Polk's predecessor, John Tyler, arranged the annexation of Texas into the United States in 1845 by a joint resolution of both Houses of Congress before Polk was sworn in. According to Mexico, the United States had torn away one of its provinces. The Mexican government rejected Polk's final offer of $35 million for California and other lands and dispatched military forces to the Rio Grande. It also rejected Texas and U.S. claims that the border extended south to the Rio Grande instead of the Nueces River.
On 23 April 1846, President Mariano Paredes announced that a state of “defensive” war existed between Mexico and the United States, in response to the violation of the Texas border by U.S. soldiers under Gen. Zachary Taylor, who marched under Polk's orders from Louisiana through Texas up to the Nueces River. On 25 April, Mexican and U.S. forces fought a skirmish between the Nueces and the Rio Grande. Eleven U.S. dragons were killed, five wounded and forty‐seven captured. It took several days for word of the skirmish to reach President Polk, who had previously decided on war even before Paredes's announcement. On 11 May, he asked Congress to acknowledge the state of war that Mexico had already announced, but did so with a resounding and controversial call: “American blood has been shed on American soil.” On 13 May, Congress strongly endorsed Polk's request, 174–14 in the House of Representatives and 40–2 in the Senate. Critics, mostly northern Whigs, condemned the president's action, asserting that he sought war to acquire more slave territory and denying that the disputed border area belonged to the United States.
Northern Mexico included two commercial centers, Santa Fe, New Mexico, and San Francisco, California. Lightly populated and distant from Mexico City, both provinces were difficult to defend. Polk met with his cabinet and formulated a remarkably ambitious strategy. He ordered U.S. soldiers to invade New Mexico, capture Santa Fe, then proceed to conquer California, where a naval squadron would assist them in securing the province. Meanwhile, General Taylor with less than 3,000 regulars would drive Mexican forces south of the Rio Grande, which the United States claimed as the international boundary. Polk assumed that if U.S. forces occupied the Rio Grande as well as key spots in New Mexico and California, Mexico would have no choice but to concede the fait accompli, and the United States would have won the war.
For their part, however, the Mexicans stood ready to fight, both for the defense of their territory and the future of their fledgling nation, which only twenty‐four years earlier had won its independence from Spain. Mexico possessed a tiny naval coast guard and on paper had a national army of more than 30,000 soldiers, over three times the U.S. Army's size at 8,500 officers and men. Numbers masked contrasts between the two armies, however. The Mexican Army was indifferently trained and unevenly equipped. Some units had enthusiastic officers, good weapons, and adequate supplies; others were deficient in all respects. Many Mexican officers held honorific commissions but knew little about military matters. The army had been involved with intrigues in the national capital, where commanders went in and out of favor with the political winds. Thus, the Mexican Army had more weaknesses than strengths. In contrast, the United States possessed an excellent navy, which could dominate the Gulf of Mexico and the California coast. The U.S. Army had competent officers, excellent weapons, good training, and the advantage of a uniform supply system. Many of its company officers (captains and lieutenants) were graduates of the U.S. Military Academy at West Point, where they received training and education in weaponry and engineering. Many Mexican officers lacked these fundamentals and had no common base of education. U.S. artillery units were particularly noted for their high quality. Although it was smaller, the U.S. Army was superior to the Mexican Army.
But the regular U.S. Army would have to be supplemented by state volunteer units. President Polk asked Congress to approve a call for thousands of volunteers, prompting an initial positive response across the country. Dozens of state volunteer regiments were recruited for federal service, giving the United States the minimum manpower it needed to fight a war over a broad expanse of territory. Approximately 73,500 volunteers served in 1846–48.
While Congress considered the issue of war, soldiers fought the first major battle above the Rio Grande at a spot called Palo Alto (near Brownsville, Texas). Leading the U.S. forces, Zachary Taylor, a professional soldier since 1808, was an intuitive commander who had seen combat against Indian tribes. Taylor's force encountered the Mexican Army of the North, with about 6,000 soldiers, under Gen. Mariano Arista on 8 May 1846. The four‐hour combat was intense and indicated the bravery and dedication of the men on both sides, with each fielding some of the best regular units of their respective armies. Several times the Mexicans delivered strong charges into well‐directed U.S. artillery fire. The Americans repulsed the attacks and Arista elected to retreat. The next day, Taylor advanced his force to locate and fight the Mexicans, who had chosen defensive positions along an old path of the Rio Grande, the Resaca de la Palma. Taylor directed an assault that broke the Mexican line. Panic gripped some Mexican units and Arista and his officers could not prevent a rout. The U.S. victory inflicted serious casualties on some of the most well‐equipped Mexican Army units, which threw away arms and supplies in their hasty withdrawal. During the two days of combat, Taylor's army suffered less than 200 killed and wounded, but the Mexicans suffered more than 600 casualties. Arista retreated again, this time south of the Rio Grande.
In the weeks to come Taylor moved his army southward, occupying Matamoros, Mexico, on 17 May and Camargo on 14 July. Pressing deeper into Mexico, he fought a battle at Monterrey in late September, captured the city, then marched on to occupy Saltillo in November. The initial phase of the war had gone just as President Polk wanted. It had also made a national hero of General Taylor, who became a likely presidential nominee for the Whig Party.
Occurring simultaneously with part of Taylor's northern Mexico campaign was the U.S. invasion of New Mexico and California. The movement of multiple U.S. units threatened the Mexicans with a cordon offensive; that is, the U.S. forces put more than one offensive action into motion at almost the same time, not allowing the Mexicans to focus only on a single campaign.
Under orders from President Polk, Col. Stephen Watts Kearny mounted the campaign against Santa Fe from a training base at Fort Leavenworth (Kansas). Kearny led a mixed force of 1,600 soldiers (including 500 regulars), departing Leavenworth on 5 June and arriving at Santa Fe on 18 August after a grueling overland march. The Mexican authorities could neither raise nor send adequate military forces to resist Kearny, who entered the city unopposed. In only a few weeks, 1,000 more volunteers were expected to arrive, allowing Kearny to take the regulars in another overland march from Santa Fe to California, while 1,200 volunteers under Alexander Doniphan, a colonel of the Missouri volunteers, moved on El Paso del Norte (modern Juarez, Mexico).
The U.S. forces completed Polk's initial plan for the war without losing a battle. Kearny arrived in California in early December 1846. By that time U.S. sympathizers had declared the “Bear Flag Republic” on 4 July. U.S. naval forces under Commodore John D. Sloat had disembarked at Monterey 7 July, and another force under Commodore Robert Stockton had occupied Los Angeles 12 August. Meeting scattered resistance, the U.S. military forces appeared to have plucked California from Mexico like a bunch of grapes. Refusing to go down without a fight, Mexicans rose against the U.S. occupation in September and fought several skirmishes, the most important of which were at San Pasqual (northeast of San Diego) on 6 December 1846, and at San Gabriel on 8 January 1847. Following that U.S. victory, however, the province was effectively in North American hands.
In the meantime, on 25 December 1846, Doniphan's men defeated a Mexican force twice their size north of El Paso. Occupying El Paso, Doniphan waited for reinforcing artillery and then marched across inhospitably dry terrain toward Chihuahua. Outside the city, a hastily trained, inadequately equipped army opposed him. Deep in enemy territory with little prospect of support, Doniphan decided to attack the Mexican Army, although his men were again outnumbered more than two to one. The battle of 28 February 1847 sent the Mexicans into headlong retreat and allowed Doniphan to occupy Chihuahua.
North Americans had invaded Mexican territory in several places and been victorious in all of the war's opening campaigns. Yet the Mexican government refused to acknowledge defeat. Until a treaty confirmed Mexico's loss of California and New Mexico, the United States could not officially claim those vast territories.
Even before word of all the North American successes in California and the Southwest had reached Washington, President Polk and Winfield Scott, general in chief of the U.S. Army, had decided to open a new phase of the war. This would require another invasion of Mexico, intended to capture Mexico City itself. If Mexico's government would not concede defeat, then Polk intended to demand concessions at bayonet point in the enemy capital. Polk ordered Scott to assemble a strong expeditionary force by taking most of Taylor's regulars and supplementing them with several thousand volunteers and a few hundred U.S. Marines. The transfer of Taylor's regulars began 3 January 1847. Eased out of the war's climactic campaign, Taylor's temper flared at Polk as well as Scott. Brilliant, and something of a perfectionist, Scott plunged into plans for the expedition, including having special wooden landing boats built to carry his soldiers from ships offshore to the Mexican beaches. The Mexican port city of Veracruz on the Gulf of Mexico was selected as the U.S. base of operations, which meant that the port would have to be taken as the first step of the campaign. Scott prepared to launch his invasion in early March.
The Mexicans were not standing idle during the U.S. preparations. Their new and controversial president, Gen. Antonio Lopez de Santa Anna, envisioned a daring gamble that might yet turn the tide of the war. Demonstrating his inspirational leadership, Santa Anna created a new field army of 25,000 soldiers though the Mexican treasury was all but empty. Several of the units were brand‐new. A number had inadequate equipment and supplies, but there was a remarkable patriotic fervor as recruits and veterans marched north to carry out their president's risky design. A captured letter informed Santa Anna of Scott's campaign plan. Therefore, Santa Anna intended to defeat Taylor's reduced army (numbering less than 5,000), encamped near Buena Vista ranch, not far from Saltillo, then return to his capital and blunt the new American threat to the heart of Mexico. To reach Buena Vista, Santa Anna sent his men over 400 miles of rough terrain in the winter. This audacious plan missed succeeding by only a narrow margin.
On 22 February 1847, Santa Anna's army attacked Taylor's near Buena Vista in a series of piecemeal assaults turned back by blistering U.S. artillery fire. The next day, the Mexicans seemed on the verge of breaking through the U.S. line when they were met by a sharp counterattack led by Col. Jefferson Davis and his Mississippi volunteers. Relying on personal inspiration, Santa Anna persuaded his men to attack again, but again the North Americans repulsed them. The two armies glared at each other on the 23rd, and that night Santa Anna decided to retreat. The road south to Mexico City was littered with discarded weapons and wounded men. Santa Anna had lost almost 40 percent of his army killed, wounded, and missing. Taylor suffered around 700 killed and wounded some 15 percent of his army.
Upon his return to Mexico City, Santa Anna appealed to patriotism and used conscription. Drawing upon new taxes and extraordinary funds taken from the Catholic Church, he formed and began training another army. Meanwhile, Scott's expedition had landed below Veracruz on 9 March, laid siege to the city, and forced its garrison to surrender twenty days later. The North Americans organized the port as their base of operations and on 8 April Scott set out on the National Road to the capital.
Santa Anna hoped to bleed Scott's army (only about 14,000 at its strongest point) as it marched inland, forcing it to fight at roadblocks and weakening it to such an extent that it would either retreat or be vulnerable to a showdown battle. On 12 April at Cerro Gordo, about fifty miles from Veracruz, Santa Anna deployed 11,000 soldiers on a natural defensive position. Scott, however, had no intention of playing to Santa Anna's strengths. Using information brought by skilled staff officers—including Robert E. Lee, P. G. T. Beauregard, and George B. McClellan—Scott maneuvered his units in such a way as to outflank the Mexican defenses, dislodging Santa Anna and pushing him back on 18 April. U.S. Casualties were about 425 killed and wounded; Mexican losses were 1,000 killed and wounded and 3,000 taken prisoner.
Although he won the first encounter with Santa Anna, Scott had numerous problems. Diseases wracked his army. One thousand North Americans lay ill in hospitals in Veracruz, and another 1,000 were ill in Jalapa, a few miles west of Cerro Gordo. Moreover, the enlistment of thousands of volunteers expired in June. Scott held up his advance at Puebla on 15 May, sending the veterans home, awaiting the arrival of reinforcements, and buying provisions for his men. His army's enrollment stood at only around 7,000. The general determined that he could not garrison a string of depots or forts along the National Road and decided to cut himself off from his supply base (Veracruz) and live off the land, rendering his army even more vulnerable than before. However, using generous terms with local mayors and townsfolk, Scott maintained unusually good relations with the Mexican populace. Some guerrillas picked at the edges of his camps and line of march, but did not weaken him appreciably.
Commanding about 10,000 soldiers, Scott proceeded to the outskirts of Mexico City. Arriving near the capital in mid‐August 1847, the general again relied on his staff officers for reliable information about terrain and enemy strengths and weaknesses. Santa Anna had mustered nearly 25,000 men, mostly new recruits and national militia leavened with only a few thousand regulars, spread all around the city. Scott chose to approach from the south, crossing terrain that Santa Anna and his subordinates considered impassable, thus creating a measure of tactical surprise that gave him an advantage. The U.S. forces launched attacks against selected Mexican strongpoints, knowing that if they suffered serious losses they might yet be overwhelmed, just as Santa Anna envisioned. In a series of major battles between mid‐August and mid‐September, Scott's soldiers fought admirably, outflanking or breaking through well‐placed and often determined Mexican defenses, such as that conducted at the Battle of Chapultepec by regulars and cadets of the Mexican military academy. Scott's army suffered more than 3,000 killed, wounded, and lost to disease during the battles for the capital. On 14 September, the victorious U.S. forces entered the plaza of downtown Mexico City, ending a remarkable military campaign. Great Britain's duke of Wellington, who had declared the expedition lost when it cut loose from its supply line, now landed Scott's achievement.
Reinforced during the weeks to come, Scott's army occupied the capital for several months, employing an effective military government while diplomatic negotiations brought the war to an official end. Other army officers also provided governmental leadership for the territory of California from 1848 to 1850. No Mexican politicians wanted to affix their names to the treaty that would give up half of their country's territory to the United States. Finally, on 2 February 1848, the diplomats agreed to the terms of the Treaty of Guadalupe‐Hidalgo. The U.S. Senate ratified the treaty on 17 March. Both sides confirmed the agreement 30 May. The United States gained all of the vast lands Polk had sought at the beginning of the war—later to become the states of California, New Mexico, Arizona, Nevada, Colorado, and Utah—taking about half of the land of Mexico but only about 1 percent of its population. The Rio Grande would form part of the boundary between Texas and Mexico. In return, the United States agreed to pay Mexico $15 million and assume all claims of American citizens against the Mexican government, about another $3 million.
In addition to the territories lost by Mexico and gained by the United States, the war produced several other important results. After expending $100 million and losing more than 10,000 military personnel (killed or died of disease), the United States became a truly continental power, stretching from the Atlantic to the Pacific, and had a vast potential for the future. Only a few months after the war, gold was discovered in California, prompting a frantic rush of settlers from all parts of the world and making California eligible for statehood in 1850, ahead of expectations. Most of the West Point graduates had acquitted themselves with distinction during the war, confirming the place of the academy in American life. The U.S. Army had fought its first overseas war and its services of supply and recruitment had worked satisfactorily enough to give the nation a victory. Scott's campaign to capture Mexico City provided notable operations for U.S. military officers to study in the future. Although only a few U.S. Marines participated in that campaign their role received favorable publicity, helping to gain continued congressional support for the Marine Corps.
Moreover, the war produced notable political consequences. By his audacious decisions and detailed direction of the war, James K. Polk broadened the powers of the president as commander in chief. The Whigs nominated Zachary Taylor and he won the presidential election of 1848. In 1852, the Whigs nominated Winfield Scott, but he lost to his former subordinate, volunteer general and New Hampshire politician Franklin Pierce, the Democratic nominee. Debates intensified over the status of slavery in the new territories, leading most immediately to the Compromise of 1850. That measure admitted California as a free state but allowed slaveowners to bring slaves into the western territories captured from Mexico. Other provisions ended the slave trade in Washington, D.C., and provided a new Fugitive Slave Law. Thus, the successful war that expanded the United States to the Pacific also further intensified the debate over issues relating to slavery and led toward the sectional crisis of 1860, secession, and the U.S. Civil War.
[See also Academics, Service: U.S. Military Academy; Army, U.S.: 1783–1865; Civil War: Causes; Marine Corps, U.S.: 1775–1865; Mexican Revolution, U.S. Military Involvement in the; Texas War of Independence.]
Ramon Alcaraz , The Other Side, trans. Albert C. Ramsey, 1850.
Justin H. Smith , The War with Mexico, 2 vols., 1919.
Robert S. Henry , The Mexican War, 1950.
Otis A. Singletary , The Mexican War, 1960.
George W. Smith and Charles Judah, eds., Chronicles of the Gringos: The U.S. Army in the Mexican War, 1968.
David M. Pletcher , The Diplomacy of Annexation: Texas, Oregon, and the Mexican War, 1973.
K. Jack Bauer , The Mexican War, 1974.
John E. Weems , To Conquer a Peace, 1974.
Robert W. Johannsen , To the Halls of the Montezumas, 1985.
Joseph G. Dawson III