Thesis: Show How Mexico Lost An Enormous Territory to the U.S. in 1848. Henry Polk was President of the U.S.A. from 1845, and was an imperialist. He wanted his country to expand by seizing Mexico's land. His goal was to try to make the States stretch from the Atlantic to the Pacific. Polk first offered 30,000,000 dollars to Mexico for California and New Mexico, but they refused. As relation between the two continued to worsen, it was in fact true that The U.S. had no real legal claim to those territories, and there were only seven hundred Americans compared to the 75,000 Spanish- s