While Polk awaited the Presidency, the trouble of Texas resurfaced.
Congress admitted Texas to the Union in a joint resolution passed the day before Polk's inauguration. Mexico was outraged. Inclusion in the United States would forever rule out the possibility of re-acquiring the lost province.
Furthermore, the boundary was in dispute. Mexico claimed that the southern boundary of Texas was the Nueces River, the Texan boundary while under Mexican rule. Americans, as well as the incoming President, claimed that the boundary of Texas was the Rio Grande River. The territory between the two rivers was the subject of angry bickering between the two nations. Soon it would serve as the catalyst for an all-out war.
President Polk's true goal was to acquire the rich ports of California. He envisioned a lucrative trade with the Far East that would revolve around San Francisco and Monterey. Great Britain also had designs on the territory, so Polk thought he would have to act fast. He sent John Slidell to Mexico with an offer. The United States would pay Mexico a combined sum of $30 million for the Texan boundary of the Rio Grande, New Mexico territory, and California.
The Mexican government was livid. They were not interested in selling the valuable territory. Instead they issued the highest diplomatic rebuke. They refused even to receive Slidell to hear his offer. The American President was enraged. He resolved to fight Mexico.
In July of 1845, Polk ordered General Zachary Taylor to cross the Nueces River with his command of 4,000 troops. Upon learning of Slidell's rejection, Polk sent word that Taylor should advance his troops to the Rio Grande River. From the standpoint of Mexico, the United States had invaded their territory. Polk hoped to defend the disputed area with armed force. He also knew that any attack on American troops might provide the impetus Congress was lacking to declare war.
Sure enough, in May of 1846, Polk received word that the Mexican army had indeed fired on Taylor's soldiers. Polk appeared before Congress on May 11 and declared that Mexico had invaded the United States and had "shed American blood on American soil!" Anti-expansionist Whigs had been hoping to avoid conflict, but news of the "attack" was too much to overlook. Congress passed a war declaration by an overwhelming majority. President Polk had his war.