Answer:
Not only was the U.S in a dispute with Mexico for years over Texas and wanted California (which was owned by mexico) so they could expand into the pacific, but Polk was a big believer in Manifest Destiny. This was an ideal held near and dear to Polk, which said the continent was for the U.S to conqueror. Getting back to it, the dispute boiled over when American troops were shot at and killed on American soil by a Mexican army. This spurred Polk on for the war he always wanted, and would defend Americas honor from Mexico.
