Final answer:
After Slidell's diplomatic mission failed, Polk provoked a military engagement by ordering troops into disputed territory, leading Congress to declare the Mexican-American War.
Step-by-step explanation:
When Mexican President Mariano Paredes refused John Slidell’s overtures, President James K. Polk took more aggressive actions leading to increased tensions and eventually, to the outbreak of the Mexican-American War. After Slidell was turned away, Polk ordered General Zachary Taylor into the disputed territory between the Nueces River and the Rio Grande, which was claimed by both Mexico and the United States.
This move resulted in a skirmish that Polk used to claim that Mexico had '“passed the boundary of United States, invaded our territory, and shed American blood upon the American soil.”' Congress, with some opposition, overwhelmingly approved Polk's request for a declaration of war against Mexico. Polk’s actions exemplified his willingness to resort to military force to achieve the territorial expansion he and many other Americans desired.