Mexican-American War

Boundary disputes turn into armed conflict

In the previous lesson, you learned about the independence of Texas from Mexico in 1836 and the subsequent annexation of Texas by the United States in 1845. The United States had offered to buy Texas from Mexico in the 1820s, an offer that Mexico had rejected. In 1845, after Congress had ratified the annexation of Texas but before annexation occurred later in December 1845, **President Polk **sent Representative John Slidell from Louisiana to negotiate with Mexico over the boundaries of Texas; you will see a further reference to Slidell in this chapter as he served as a **Confederate **emissary to the United Kingdom during the Civil War.

Texas claimed that its boundaries reached the **Río Grande **and also included parts of what are now New Mexico and Colorado, while Mexico contended that the southern boundary of Texas reached the Río Nueces, a distance of at least 150 miles, and did not include the northern and western territories in contemporary New Mexico and Colorado. Slidell sought to convince Mexico to settle these Texas boundary disputes in exchange for the United States paying Mexican claims against the United States; President Polk also instructed Slidell to attempt to purchase **California **for an additional $25 million. Mexico again rejected American overtures to purchase Texas as well as California. President Polk then ordered **General Zachary Taylor to move forces into the “disputed territory” between the Río Nueces and the Río Grande; **after Mexican forces fired upon US soldiers, Polk sought and received a **declaration of war **from Congress in May 1846.

Military success, political controversy

While the United States ultimately prevailed militarily in the Mexican-American War, political controversies and disputes significantly impacted the country, often building upon existing disagreements about **manifest destiny **and the expansion of slavery into new territories. American forces captured major Mexican cities, including the port of Veracruz, and forced the Mexican government to sign the **Treaty of Guadalupe-Hidalgo **in February 1848, ceding hundreds of thousands of square miles of Mexican territory to the United States. While many Americans celebrated victory and welcomed the addition of these new territories, political controversies during and after the war reinforced existing partisan and sectional differences.

In August 1846, Pennsylvania Democratic Representative **David Wilmot **sparked considerable controversy in Congress when he introduced an amendment to a military funding bill that prohibited the expansion of slavery into any Mexican territory newly acquired by the United States. While the Wilmot Proviso was rejected in the House of Representatives and never became law, anti-slavery forces throughout the country cheered this proposal while southern slaveowners condemned it. Wilmot was neither an abolitionist nor opposed to the institution of slavery where it already existed; when he explained the rationale for his proposal, he argued that “I would preserve for free white labor a fair country, a rich inheritance where the sons of toil of my own race and color, can live without the disgrace which association with negro slavery brings upon free labor.”

In December 1847, a then obscure Illinois Whig Representative, Abraham Lincoln, introduced a series of **spot resolutions **demanding that President Polk prove to the American people that the first American soldiers killed by Mexican forces in 1846 were in fact still clearly on American soil and not on Mexican soil as the Mexican government contended. These spot resolutions were largely ignored in Congress, an unsurprising development considering that American forces were then rapidly approaching Mexico City.

What happens now?

Once the United States claimed the new territories that it referred to as the Mexican cession, an area encompassing contemporary Arizona, California, most of Colorado, Nevada, New Mexico, expanded boundaries for Texas, Utah, and even a small part of Wyoming, it immediately confronted challenges regarding the status and treatment of thousands of previous Mexican citizens and indigenous societies. Furthermore, controversies over the potential expansion of slavery into these new territories had not been resolved.

In American politics, Zachary Taylor’s fame from the Mexican-American War propelled him to the Whig nomination as well as victory in the election of 1848 which also featured former president Martin Van Buren nominated by the Free Soil Party, a temporary alliance of disaffected Whigs and northern Democrats opposed to the expansion of slavery into these newly acquired territories. Many Free Soil supporters founded and supported the Republican Party in the early 1850s, subscribing to the summarizing principles of the 1848 Free Soil platform - “Free Soil, Free Speech, Free Labor, and Free Men.”