We paid for California!
the United States paid Mexico $15,000,000 cash and assumed some $3,250,000 more in claims of American citizens on the Mexican government. Considering the facts that California was scarcely under Mexican control at all and might have been taken at any moment by Great Britain, France, or Russia; that New Mexico was still the almost undisturbed home of Indian tribes; that the land from the Nueces to the Rio Grande was almost a desert; and that the American troops were in possession of the Mexican capital, the terms offered Mexico were very generous. Polk was urged by many to annex the whole country of Mexico to the United States, but he refused to consider such a proposal.
Just a little real estate transaction is all.