The relationship between the United States and Mexico remained tense for many decades with several military encounters along the border. The war engendered a deep and long-lasting feeling of resentment on Mexicans toward Yankees.
The relationship between the US and Mexico became "sour" when Congress voted to annex Texas. To Mexico, it was an act of war. Also, Texas and Mexico couldn't decide on a border. Texas wanted it to be the Rio Grande, but Mexico wanted it to be the Nueces River, about 150 miles northeast of the Rio Grande.
On April 25, 1846, the US to declared war on Mexico following the Mexican cavalry's attack on a group of U.S. soldiers in the disputed zone under the command of General Zachary Taylor, killing about a dozen.
The U.S.-Mexican War had a tremendous impact on the history of both countries. For the United States, this was the nation's first foreign war fought almost entirely on foreign territory. It involved multiple armies, long supply lines, a very large-scale amphibious landing of troops, and it provided the first experience of occupying a foreign capital and establishing a military government for an alien population.
For Mexico, the war was a series of tragedies. Besides the thousands of military and civilian deaths directly attributed to battles, the war left tens of thousands of orphans, widows and disabled.
To learn more, take time to visit the following sources below:
history.state.gov
www.history.com
www.pbs.org
www.historyguy.com