I n 1845, the Republic of Texas was annexed by the United States with the almost unanimous consent of its citizens. This was the underlying cause of the Mexican war. Mexico objected to the annexation, holding that Texas was still part of their country, even though Texans had fought and won their independence nearly ten years earlier and had been formally recognized as a sovereign nation by the United States, Great Britain, France, and other countries.

Some historians seek to blame the United States for the war, but it was clearly the fault of Mexican leaders' unwillingness to concede the loss of Texas. Their refusal to negotiate with the United States in respect to the independence of Texas and its border pushed the two nations to the brink of war.

On April 25, 1846, after Texas joined the United States, a large body of Mexican troops crossed the Rio Grande into Texas and ambushed a small group of American soldiers, killing sixteen and taking the remainder prisoner. American troops under the command of General Zachary ("Old Rough n' Ready") Taylor had taken up a position on the north bank of the Rio Grande in the spring of 1846, after Mexican President Mariano Paredes refused to negotiate with the United States and threatened an invasion of Texas.

By serving in the war with Mexico, U.S. soldiers were defending the right of a free people, that is the citizens of the Republic of Texas, to determine their own destiny, namely to become part of the United States.