Why was President Polk we need to go to war with Mexico

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.