The story of how Texas won its independence in 1836 is told in its own section, below. From 1836 to 1845, Texas led a precarious existence as an independent republic, officially recognized by Great Britain, France, and the United States, but desiring to join the Union. When Texas did finally enter the Union as the 28th state on December 29, 1845, war with Mexico soon followed.
Here, as elsewhere, it is important to remember that hindsight varies with the times. Sometimes certainties are merely our own unconscious prejudices. It is tempting to see that war as simple American aggression against a relatively blameless neighbor. Indeed, many saw it that way at the time, and it shocked those who thought of the United States as a new beginning in world affairs. Emerson said it as part of a poem:
“But who is he that prates Of the culture of mankind, Of better arts and life? Go, blind worm, go, Behold the famous States Harrying Mexico With rifle and with knife.”
Most Whigs opposed the war, wanting to strengthen the economy with industrialization, rather than expand it. Most Democrats, on the other hand, supported expansion as the nation’s “manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.” At any rate it wasn’t as simple as President Polk made it out to be in his message to Congress, in which he said that Mexico had “shed American blood upon American soil.”
But whatever the ultimate rights and wrongs of the situation, Mexican actions in crossing the Rio Grande and attacking American soldiers had precipitated a state of war, and then the war had to be fought and won. Once it came to war, the situation had a logic of its own.
Mexico was in political chaos, insolvent, with no conceivable way to pay the many outstanding financial claims against it except perhaps to cede the territories of New Mexico and California in return for the American government undertaking to pay Mexico’s claims. These territories, though long part of Mexico, were largely frontier lands, unsettled, ungoverned, and unprotected, with Americans, rather than Mexicans, constituting much of the non-native population. The fear was that if the United States did not take possession, they might fall to the British Empire, then in its heyday.
What’s more, the people of Northern Mexico didn’t necessarily object to the transfer of sovereignty. In the 25 years since Mexico became independent in 1821, it had become less and less able to defend the northern half of the country. Comanche, Apache, and Navajo Indians, especially Comanche, engaged in large-scale raids hundreds of miles into the country, stealing livestock and killing thousands of people. When American troops entered northern Mexico in 1846, they found a demoralized people who offered little resistance, perhaps preferring their new masters to their old, perhaps not, but at any rate appreciating the greater security that the American military presence promised.
The genesis of the war was simple enough. Texas had won its independence in 1836 on the battlefield at San Jacinto, in which Sam Houston’s army captured Mexican president (and general) Antonio Lopez de Santa Anna. Santa Anna had signed the Treaties of Valasco, which recognized the Rio Grande as the boundary of the Republic of Texas, but the government of Mexico repudiated the treaties, did not concede the Texas independence, and insisted that in any case the Nueces River was the limit of the territory that it controlled de facto.
For political reasons, the Congressional resolution annexing Texas deliberately omitted any mention of the Rio Grande boundary, but when the Republic of Texas became the 28th state, the United States inherited its territorial claims. President Polk sent General Zachary Taylor to Texas, and by October 3,500 Americans were on the Nueces River, ready to occupy the disputed land. In November, Polk sent a secret representative to Mexico City to offer $25 million for the Rio Grande border in Texas, to forgive the $3 million owed to U.S. citizens for damages caused by the Mexican War of Independence [from Spain] 25 years earlier, and to pay another $25 to $30 million in exchange for the two territories of Alta California and Santa Fe de Nuevo México.
Mexicans refused to deal. When president Jose Joaquin de Herrera considered receiving the president’s envoy, he was accused of treason and deposed, and a more nationalistic government publicly reaffirmed Mexico’s claim to Texas.
At this, President Polk ordered General Zachary Taylor south to the Rio Grande, about 150 miles south of the Nueces. Mexico demanded that Taylor withdraw. Instead, Taylor constructed a makeshift fort on the banks of the Rio Grande opposite the city of Matamoros. On April 25, 1846, a 2,000-strong detachment of Mexican cavalry attacked a 70-man U.S. patrol, routing it and killing 16 American soldiers. American blood had been shed, to be sure. On American soil? Polk thought so, or pretended to think so, and sent a message to Congress saying so. Congress approved a declaration of war on May 13.
Santa Anna persuaded the Americans that he would work to sell the contested territory at a reasonable price, and he persuaded the Mexicans that he just wanted to fight for his country. Or course, as soon as he got command of an army, he double-crossed everybody, declaring himself president and trying to fight off the Americans.
In May, Taylor and 2,400 troops defeated 3,400 Mexicans in the Battle of Resaca de la Palma, in fierce hand to hand combat. Taylor’s troops crossed the Rio Grande and took the city of Monterrey, then in February, 1847, held the mountain pass at Buena Vista against an attack by 15,000 Mexican troops led by Santa Anna personally. A second army under General Winfield Scott was transported by sea to the port of Veracruz, and on March 9, 1847, Scott performed the first major amphibious landing in U.S. history, using specially designed landing craft and 12,000 volunteer and regular soldiers to offload supplies, weapons, and horses. Veracruz surrendered after 12 days. Scott then marched westward toward Mexico City, defeated Santa Anna at Cerro Gordo, captured Puebla, the second largest city in Mexico, and successfully stormed Mexico City in August.
Meanwhile, a U.S. cavalry force reinforced by a Pacific fleet had invaded western Mexico, lest Britain seize the area. California was won by army and navy operations, culminating in the treaty of Cahuenga on January 13, 1847.
President Polk achieved his goal of American territorial expansion to the Pacific coast. The Treaty of Guadalupe Hidalgo forced Mexico to cede Alta California and New Mexico to the United States in exchange for $15 million and the assumption by the United States of $3.25 million of debt owed by the Mexican government to U.S. citizens. Altogether, adding up territory lost by the secession of Texas, the territory ceded after the war, and the Gasden Purchase of 1853, Mexico’s land-mass was reduced by more than 55%. However, the land lost was mostly empty of Mexicans. Alta California contained only about 14,000 Mexicans; Nuevo México, fewer than 60,000. Of those, the great majority chose to remain where they were rather than relocate to the south.
With time and politics, the war came to be seen in the North as a Southern plot to gain new territory geographically suited for the expansion of Slavery, and that view, though not complete, was not wrong. The question of whether the newly acquired territories were to be free or slave brought the Union closer to Civil War. It might have come in 1850 – almost did – but in 1850 political opinion in the North had not been hardened by a decade of ever-increasing defiance, insolence, and invective hurled at it day by day by representatives of the South. That defiance stemmed from a sense of injustice, as the Southerners saw what looked to them like a conspiracy to deprive them of the spoils of a war that had been largely fought by Southern troops. President Ulysses S. Grant, who served in Mexico under General Taylor as a young army lieutenant, said in his Memoirs: “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.”