What were president polk's reasons for going to war with mexico??

A few days before Polk made this request to Congress, he learned that Mexican forces had crossed the Rio Grande and killed eleven US soldiers. On April 24, 1846, the Mexican cavalry attacked and captured the personnel of one of the US detachments at the Rio Grande. After this border conflict, battles between the Mexican and US forces broke out in Palo Alto and Resaca de la Palma.

Polk considered this a casus belli and in a message to the Congress of 11 May 1846 he asserted that Mexico had "invaded our territory and shed US blood on US soil". He did not dwell on the fact that there was a dispute over the territory in question. A number of congressmen expressed doubts about the version provided by Polk, but Congress passed the war declaration by an overwhelming majority, with numerous Liberals (Whigs) intimidated by the fact that their opposition would cost them politically. The war was declared on May 13, 1846. The Congressmen originally from the North of the United States and the Liberals in principle opposed the war, while those originating from the South of the United States and the Democrats tended to support it. Mexico declared war on May 23rd.