When the United States annexed Texas, it also took up Texas's claim that its border was the Rio Grande. Mexico said the border was the Nueces River, farther north. President Polk also wanted California and offered to buy it; Mexico refused to talk.
Key idea
In 1846 Polk sent troops under Zachary Taylor into the disputed land, and fighting broke out. Congress declared war on May 13, 1846. U.S. armies won in northern Mexico, took California and New Mexico, and captured Mexico City (September 1847). In the Treaty of Guadalupe Hidalgo (February 2, 1848), Mexico gave up the Mexican Cession (California and most of the Southwest) and accepted the Rio Grande border, and the U.S. paid \$15 million.
Why it works
Polk told Congress that Mexico had "shed American blood upon the American soil." Critics answered that the soil was disputed and that the President had provoked the war. Many Whigs and abolitionists also feared the real goal was new land for slavery. In the House, David Wilmot proposed the Wilmot Proviso (1846), which would have banned slavery in any land taken from Mexico. It passed the House but failed in the Senate, and the vote split along sectional lines rather than party lines.
Worked example: Why the Proviso mattered
The Wilmot Proviso never became law. Why do historians still count it as a turning point?
- On most questions, Congress voted by party: Democrats against Whigs.
- On the Proviso, Northern Democrats and Whigs mostly voted for it, and Southern members of both parties against it.
- That showed slavery in the West could split both national parties along sectional lines.
- The question it raised, slavery in the new land, led straight to the Compromise of 1850.
Answer: It split Congress by section, not by party, and put slavery in the new land at the center of politics.
Common mistake
The war was not widely popular everywhere. Many Whigs, abolitionists and others opposed it. A first-term Whig congressman, Abraham Lincoln, demanded that Polk name the "spot" where American blood was shed; Henry David Thoreau spent a night in jail for refusing to pay a tax and later wrote "Resistance to Civil Government" (known as "Civil Disobedience"). It was popular with many others, especially in the South and West, and volunteers enlisted in large numbers. Check: a war can be popular in one region and opposed in another.
Tens of thousands of Mexican citizens lived in the ceded land. The treaty promised them U.S. citizenship and protection of their property, but in the years that followed many lost land in American courts. Native nations of the region were not parties to the treaty at all.