Texas thus joined the United States as a slave state in 1845.[193]
Adams had served with James K. Polk in the House of Representatives, and Adams loathed the new president, seeing him as another expansionist, pro-slavery Southern Democrat.[194] Adams favored the annexation of the entirety of Oregon Country, a disputed region occupied by both the United States and Britain, and was disappointed when President Polk signed the Oregon Treaty, which divided the land between the two claimants at the 49th parallel.[195] Polk's expansionist aims centered instead on the Mexican province of Alta California, and he attempted to buy the province from Mexico. The Mexican government refused to sell California or recognize the independence and subsequent American annexation of Texas. Polk deployed a military detachment led by General Zachary Taylor to back up his assertion that the Rio Grande constituted the Southern border of both Texas and the United States. After Taylor's forces clashed with Mexican soldiers north of the Rio Grande, Polk asked for a declaration of war in early 1846, asserting that Mexico had invaded American territory. 