The line was now drawn, and neither side budged. The expected explosion happened on April 25, 1846. A patrol of US cavalry troopers, led by Captain Seth Thornton, was ambushed by the Mexicans. Eleven Americans were killed, and fifty-two, including Thornton, were captured. With the death of Thornton's men, the spark was lit, and the Mexican-American War had begun. Within hours, Mexican cannon in Matamoros and American guns in Fort Texas were exchanging fire over the Rio Grande. Battles followed at Palo Alto and Resaca de la Palma. Within weeks, the US launched a full-scale invasion of Mexico.
When the Mexican War began, it was carried along on a wave of patriotism and enthusiasm, as people eagerly volunteered and supported the effort to exact revenge on the "treacherous Mexicans" who had, they were told, crossed into American territory and killed our troops in cold blood.
But as the war went on, the body count got higher, and the economic effects began to be felt, enthusiasm waned. The Whig party, led by Senator Henry Clay (who had lost a son in the war) went on the attack, referring to the conflict as "Mr Polk's War". Not only did the veracity of Polk's account begin to be questioned and accusations made that the country had been lied to by the President to lead us into an unnecessary conflict, but doubts were raised about the entire war and its actual purpose. The US was not fighting for freedom or liberation for the Mexicans, critics concluded-we were simply bullying a weaker neighbor, who couldn't fight back, into giving us a large tract of land that we wanted for ourselves. Others opposed the war as being a mere pretext to expand the institution of slavery into new territory and to strengthen the South in its already-heated conflict with the North over the "peculiar institution".
The American military emphasis in the Mexican War was directed towards Mexico City, to force the capitulation of the Mexican Government. But the real aim of the war was the capture of the Northern territories of New Mexico and California. It was General Stephen Kearny who was given this task. He had only a handful of troops: Polk expected that the American settlers and Mexicans living in these areas would welcome the US as a liberator, and there would be little or no fighting.
Kearny's strategy was to first take the Mexican city of Santa Fe and then move on to California. The province of New Mexico promptly capitulated without a shot. Kearny then met with the famed scout Kit Carson, who informed him that the American settlers in California had rebelled against Mexico and declared their own independent Bear Flag Republic under General John Fremont, which would welcome annexation. Hearing this, Kearny left most of his troops behind, thinking they would not be needed, and moved on to California. But once there, he unexpectedly ran into stubborn resistance from Mexican guerrillas. By the time Kearny reached Sacramento, he had himself been wounded and his troops had suffered heavy losses. It took reinforcements and fierce fighting at Sacramento and Los Angeles before California was conquered, and Kearny announced its occupation by the United States.
The rest of the US Army, meanwhile, under Generals Zachary Taylor and Winfield Scott, advanced through Mexico and won a series of battles. American troops entered Mexico City on September 14, 1847. The war which President Polk had assumed would last only a matter of weeks, had taken almost two years. The US lost some 6,000 men wounded and killed, making the Mexican-American War, in terms of percentage of troops engaged, the deadliest conflict ever fought by the US.
The victorious Americans, however, now faced an unexpected dilemma. Although the war had started as a way for the US to seize the Northern Territories, a sizable faction of Democrats now argued that, with the Mexican Government gone and the country under American military and political control, there was nothing to stop them from simply annexing the entire country and turning it all into US territory. In particular, this argument was strongly pushed by the southern states, who foresaw that incorporating a large number of new slave territories would tip the political balance permanently on their side.
Other Democrats, including Polk, opposed this idea-not out of any moral scruples about slavery or even about conquering a weaker neighbor, but for reasons of practicality. The northern part of Mexico was only sparsely populated, and was essentially empty turf just waiting for American settlers to come pouring in and occupy it. But southern Mexico had almost seven million Mexicans living there, and Polk had no desire at all to incorporate what he viewed as an inferior people into the United States. It could also be anticipated that many of these Mexicans would resist annexation, which would lead to a long, bloody and expensive guerrilla war which the US did not want to fight. Polk made his decision: he instructed his envoy, Nicholas Trist, to settle on terms that obtained northern Mexico for the US and left the remainder to the Mexicans.
But when Trist arrived to negotiate a peace treaty, he found that there was nobody in Mexico City to negotiate with: Santa Anna had taken the remnants of his army and fled to the hills, and the Mexican Government had fallen apart in faction fighting. Nobody wanted to bear the responsibility for surrendering to the Americans and signing away half of their country.
Eventually, the remains of the Mexican Government appointed an "Interim President" to negotiate a treaty. But just as Trist began his talks, a message arrived from an impatient and frustrated President Polk, abruptly ordering him to return to Washington DC. Trist, knowing that Polk was not aware of the situation, decided to simply ignore Polk's order, and to carry out his negotiations anyway.
The Treaty of Guadalupe Hidalgo granted the territories of New Mexico and California to the US, set the border with Texas at the Rio Grande, and provided for an American payment of $15 million to Mexico-half of the original pre-war offer. Unknown to the diplomats in Mexico, in the week before the treaty was signed, gold was discovered in California.
The Mexican War had other effects. Polk, stung by the unpopularity of the war, declined to run for a second term and ended his political career. General Zachary Taylor, seen as a war hero, ran on the Whig ticket in 1848. He died in the summer of 1850 after less than a year and a half in office. General Winfield Scott ran for President several times, and lost. He was still the commanding General of the US Army when the Civil War broke out, but was soon replaced by General Ulysses S Grant, who had served under him in Mexico.
The new American territories in New Mexico, Arizona, California, Utah, Nevada and Colorado brought the slavery issue once again to the fore of American politics. In the Compromise of 1850, Congress agreed to admit California as a "free" state, and to allow the other new states to decide by election whether to be "slave" or "free". As part of the compromise, the Fugitive Slave Act was also strengthened. Rather than defuse the slavery issue, the Compromise of 1850 intensified it. Within ten years, the US would be embroiled in civil war.
The Treaty of Guadalupe Hidalgo, meanwhile, influences the two nations even today. The United States interpreted the provisions of the treaty as allowing for the free movement of Mexican citizens into the US, to allow families who had been separated by the new border to visit each other. The American-Mexican border was virtually open for almost 100 years, until the US began security measures in response to the Second World War and the threat of German spies or saboteurs entering the US through Mexico. Since that time, the movement of Mexican citizens and immigrants across the border into the US has become a hot-button political issue that strongly effects domestic politics as well as relations between the two Republics.
Today, one of the few remaining reminders of the Mexican War in the United States is the Palo Alto National Battlefield, where General Zachary Taylor scored his first win over the Mexican Army. Not far away is Resaca de la Palma. At the edge of Brownsville, just across the Rio Grande from Mexico, some of the remnants of the earthenworks ditches from Fort Texas still remain, but are inaccessible because of the border fence. Across the border in Matamoros is the Fort Casamata Museum.