Answer:
What happened was the U.S. annexed Texas as part of the U.S and that made Mexico mad but then the U.S. insisted that Rio Grande river was the border of Texas but Mexico did not want it to be Rio Grande River. The Americans already wanted the land of California And New Mexico and tried to offer money to the Mexicans but they resisted. So the States knew that the only way to gain the land was by war and they patiently waited to let Mexico start the war.