5.3 Manifest Destiny and the Mexican-American War
The U.S. annexation of Texas quickly led to diplomatic trouble with Mexico. The Mexicans' anger over the annexation and the newly elected President Polk's desire to expand the nation to the Pacific Ocean combined to bring both sides to the edge of war.
Conflict with Mexico
Upon taking office in 1845, President Polk dispatched John Slidell as his special envoy to the government in Mexico City. Polk wanted Slidell to (1) persuade Mexico to sell the California and New Mexico territories to the United States and (2) settle the disputed Mexico-Texas border. Slidell's mission failed on both counts. The Mexican government refused to sell California and insisted that Texas's southern border was on the Nueces River. Polk and Slidell asserted that the border lay farther to the south, along the Rio Grande.
Immediate Causes of the War
While Slidell waited for Mexicos response to the U.S. offer, Polk ordered General Zachary Taylor to move his army toward the Rio Grande, across territory claimed by Mexico. On April 24, 1846, a Mexican army crossed the Rio Grande and captured an American army patrol, killing 11. Polk used the incident to justify sending his prepared war message to Congress. Northern Whigs opposed going to war over the incident and doubted Polk's claim that American blood had been shed on American soil. Whig protests were in vain. A large majority in both houses approved the war resolution.
Military Campaigns
Most of the war was fought in Mexican territory by small armies of Americans. Leading a force that never exceeded 1,500, General Stephen Kearney succeeded in taking the New Mexico territory and southern California. Backed by only several dozen soldiers, a few navy officers, and American civilians who had recently settled in northern California, John C. Frémont quickly overthrew Mexican rule in the region in June 1846. He proclaimed California to be an independent republic. Because the new republic's flag included a Calitornia grizzly bear, it became known as the Bear Flag Republic.
Meanwhile, Zachary Taylor's force of 6,000 men drove the Mexican army from Texas, crossed the Rio Grande into northern Mexico, and won a major victory at Buena Vista (February 1847). President Polk then selected General Winfield Scott to invade central Mexico. The army of 14,000 under Scotts command succeeded in taking the coastal city of Vera Cruz and then captured Mexico City in September 1847.
Consequences of the War
For Mexico, the war was a military disaster from the start, but the Mexican government was unwilling to sue for peace and concede the loss of its northern lands. Finally, after the fall of Mexico City, the government had little choice but to agree to U.S. terms.
Treaty of Guadalupe Hidalgo (1848): The treaty negotiated by diplomat Nicholas Trist with Mexico consisted of terms favorable to the United States:
• Mexico recognized the Rio Grande as the southern border of Texas.
• The United States took possession of the former Mexican provinces of California and New Mexico-the Mexican Cession. For these territories, the United States paid $15 million and assumed responsibility for any claims of American citizens against Mexico.
In the Senate, some Whigs opposed the treaty because they saw the war as an immoral effort to expand slavery. A few Southern Democrats disliked the treaty for opposite reasons. As expansionists, they wanted the United States to take all of Mexico. Since this land was south of the line established in the Missouri Compromise dividing slave and free territory, it was a region where slavery could expand into. Nevertheless, the treaty was finally ratified in the Senate.
Wilmot Proviso: The issue of slavery made the U.S. entry into a war with Mexico controversial from start to finish. In 1846, Pennsylvania Congressman David Wilmot proposed that an appropriations bill be amended to forbid slavery in any territory acquired from Mexico. This prohibition appealed to many voters and lawmakers who wanted to preserve the land for White settlers and protect them from having to compete with enslaved labor. The Wilmot Proviso, as it was called, passed the House, where the populous Northern states had greater power, twice. Both times, it was defeated in the Senate, where Southern states had greater influence.
Prelude to Civil War?: By increasing tensions between the North and the South, did the war to acquire territories from Mexico lead inevitably to the American Civil War? Without question, the acquisition of vast western lands did renew the sectional debate over the extension of slavery. Many Northerners viewed the war with Mexico as part of a Southern plot to extend the "slave Power" Southerners realized they could not count on Northerners to accept the expansion of slavery. The Wilmot Proviso was the first round in an escalating political conflict that led ultimately, though not inevitably, to civil war.
