Elected on a platform promising to expand the United States westward, James K. Polk of Tennessee wasted no time provoking a war with the Republic of Mexico over territory. With his eye's on California and New Mexico, Polk sent an envoy to Mexico City with an offer to purchase the territory. At the same time, he ordered American troops into disputed territory near the border, hoping to taunt Mexican soldiers into an attack. When the attack failed to materialize, Polk decided to declare war on Mexico anyway. On 9 May 1846, however, the day he planned to send his declaration to Congress, Polk learned that eleven U.S. soldiers had been killed by Mexicans in the disputed area. He quickly rewrote his declaration of war, excerpted here:

The existing state of relations between the United States and Mexico renders it proper that...