The Mexican War between the United States and Mexico began with a Mexican attack on American troops along the southern border of Texas on Apr. 25, 1846. Fighting ended when U.S. Gen. Winfield Scott occupied Mexico City on Sept. 14, 1847; a few months later a peace treaty was signed (Feb. 2, 1848) at Guadalupe Hidalgo. In addition to recognizing the U.S. annexation of Texas defeated Mexico ceded California and , New Mexico (including all the present-day states of the Southwest) to the United States.
As with all major events, historical interpretations concerning the causes of the Mexican War vary. Simply stated, a dictatorial Centralist government in Mexico began the war because of the U.S. annexation (1845) of Texas, which Mexico continued to claim despite the establishment of the independent republic of Texas 10 years before. Some historians have argued, however, that the United States provoked the war by annexing Texas and, more deliberately, by stationing an army at the mouth of the Rio Grande. Another, related, interpretation maintains that the administration of U.S. President James K. Polk forced Mexico to war in order to seize California and the Southwest. A minority believes the war arose simply out of Mexico's failure to pay claims for losses sustained by U.S. citizens during the Mexican War of Independence.
Zachary Taylor began his advance on Monterrey. He reached that fortified town, which had a garrison of more than 10,000 troops, on September 19 and began his attack on the morning of September 21. With about 2,000 men, Gen. William J. Worth captured the road between Monterrey and Saltillo and by noon was storming Federation Hill. Six companies of Texas Rangers charged up the hill, seized the enemy artillery, and turned the cannon on retreating Mexican forces. On the opposite side of the city a diversionary attack penetrated the town, despite much confusion. On September 22 the Americans rested, but they resumed the attack the next day. After bloody street-to-street fighting, the Mexican general Pedro de Ampudia requested and was granted a truce. On September 25 he was permitted to withdraw his forces from the city, and an 8-week armistice was agreed upon. Total Mexican casualties were estimated at 367. The Americans had 368 wounded and 120 killed.
Taylor occupied Matamoros on May 18 but then delayed for several months before moving south. He was apparently waiting for transportation promised him by the U.S. government, though his critics branded him inept. In July he moved his base up the Rio Grande to Camargo, but it was only in August that Taylor began planning the attack on Monterrey.By that time American strength on the Rio Grande had swollen to nearly 20,000 troops, nearly all volunteers. The principal military problem was logistical support of such a quickly expanded force. The Americans were susceptible to subtropical diseases and found it difficult to maintain sanitary conditions in the camps. Fevers, dysentery, and general debility were rampant, and the mortality rate from sickness was alarming. A determined Mexican attack in July or August would have proven disastrous to the Americans.
Final battle in Northern Mexico (Battle of Buena Vista)
In January 1847, Santa Anna moved north with about 20,000 men to dislodge Taylor. Dispatches captured by the Mexicans had revealed that most of Taylor's forces were being withdrawn to take part in Gen. Winfield Scott's proposed landing at Veracruz. Word of Santa Anna's approach reached Taylor on February 21, and although outnumbered almost three-to-one, he took up a position at the hacienda of Buena Vista, a few miles from Saltillo. The Mexican attack began on February 22, when troops led by Ampudia gained an advantage and forced the Americans to abandon important defensive positions. The next morning the main Mexican force nearly overcame the U.S. defense. However, a dramatic charge led by Col. Jefferson Davis about noon and a determined artillery advance under Capt. Braxton Bragg finally saved the day for the Americans. Their casualties numbered about 700, but the Mexican losses were about 1,800. Santa Anna withdrew that night and moved south to intercept Scott's invasionary force. No further fighting occurred in northern Mexico, but Taylor remained in command of a small force there until he returned to the United States in November 1847.
Gen. Philip Kearny established a civil government with Charles Bent, a Santa Fe trader from Missouri, as governor. He then divided his command into three groups: one, under Sterling Price, to occupy New Mexico; a second, under Alexander William Doniphan, was intended to capture Chihuahua; the third, under Kearny, headed for California. Price faced rebellion in New Mexico in January 1847. Bent was murdered at his home in Taos during the rebellion. Price fought three battles against rebels, many of whom were Pueblo Indians, and by mid-February had the revolt under control.
Doniphan and 1,000 Missouri Volunteers marched to the city of Chihuahua. They were sent to join Gen. Wool. Doniphan was victorious in two battles with Mexicans and victoriously marched into the city on March 2nd. He and his troops rested for 6 six weeks before joining Wool at Saltillo.
The Bear Flag Revolt began in June 1846, when the almost bloodless “Battle” of Olompali took place. 62 U.S. troops were placed along side the American rebels. While this revolt was happening, Commodore John D. Sloat heard rumors of the war in California. So he went northward in his flagship USS Savannah, landing in Monterey on July 1st or 2nd. Waiting almost a week before raising the U.S flag from the customhouse, he may have been waiting to see if there really was a war. U.S flags were raised at Yerba Buena on July 9th, Sonoma on July 9th, and New Helvetia on July 11th. Central California had been conquered. The U.S. Army had “marched all over California from Sonoma to San Diego and raised the American flag without opposition or protest.”
Despite the objections of the abolitionists, the war received enthusiastic support in all sections of the United States and was fought almost entirely by volunteers. The army swelled from just over 6,000 to over 115,000. Of this total approximately 1.5 percent were killed in the fighting, and nearly 10 percent died of disease; another 12 percent were wounded or discharged because of disease or both. For years afterward, Mexican War veterans continued to suffer from the debilitating diseases contracted during the campaigns. The casualty rate was thus easily over 25 percent for the 17 months of the war; the total casualties may have reached 35-40 percent if later injury- and disease-related deaths are added. In this respect the war was the most disastrous in American military history.
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 unsatisfactory result of Nicholas Trist's unauthorized negotiations. It was reluctantly 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 Texas north of the Rio Grande formalized the addition of 3.1 million sq km (1.2 million sq mi) 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.