The war between Mexico and the United States that broke out in 1846 set the tone for international relations in the Americas. It was the first, and so far remains the only, formal war waged by the United States against a Latin American country. It was long assumed that the annexation of Texas (cf. Texan secession) was the key cause of the war. In fact, the underlying issue was the disputed western frontier of the new state of Texas, and thus the frontier between Mexico and the United States.

The US President James K. Polk at first hoped to give the impression of settling the co…