The mexican war (1846-1848)
- Polk dispatched a special minister John Slidell to try to buy off the Mexicans
- Mexicans leaders rejected Slidell's offer to purchase the disputed territories
- On January 13, 1846, as soon as he heard the news, Polk ordered Taylor's army in Texas to move across the Nueces River where it had been stationed to the Rio Grande.
- Mexican troops crossed the Rio Grande and attacked a unit of American soldiers.
- Many argued that the hostilities with Mexico were draining resources and attention away from the more important issue of the pacific Northwest
- Opponents claimed that Polk had settled for less than he should have because he was preoccupied with Mexico
- Polk ordered Taylor to cross the Rio Grande, seize parts of Northeastern Mexico and then to Mexico City itself
- Polk ordered other offensives against New Mexico and California
- In the summer of 1846, a small army under Colonel Stephen W. Kearny captured Santa Fe with no opposition.
- The United States now controlled the two territories for which it had given to war; New Mexico and California
- A new Mexican government took power and announced its willingness to negotiate a peace treaty.
- Polk sent a presidential envoy, Nicholas Trist, to negotiate a settlement.
- On February 2, 1848, he reached agreement with new Mexican government on the Treaty of Guadalupe Hidalgo, which Mexico agreed to cede California and New Mexico to the United States and acknowledge the Rio Grande as the boundary of Texas.
- United States had to assume any financial claims its new citizens had against Mexico and to pay the Mexicans $15 million.
- The president submitted the Trist treaty to the senate which approved it by a vote of 38 to 14