Final answer:
James K. Polk believed that Mexico was going to sell Upper California and New Mexico to the United States.
Step-by-step explanation:
James K. Polk believed that Mexico was going to sell Upper California and New Mexico to the United States.
Polk's belief was based on his campaign promise to annex Texas as a new slave state and take control of more land for the United States. He fulfilled this promise by gaining Oregon and provoking a war with Mexico. By 1848, the United States had expanded its territory from the Atlantic to the Pacific.
Learn more about Mexican-American War