Final answer:
John Slidell was sent to Mexico in a diplomatic mission to resolve the border dispute between the United States and Mexico and negotiate the purchase of a substantial portion of Mexican territory for the United States. However, his mission was met with backlash and tensions escalated into the Mexican-American War.
Step-by-step explanation:
John Slidell was sent to Mexico in a diplomatic mission in an attempt to resolve the border dispute between the United States and Mexico. His primary purpose was to negotiate the purchase of a substantial portion of the Mexican northwest, including California and New Mexico, for the United States. However, this offer was met with outrage from the Mexican government and the Mexican press, leading to Slidell being sent away and the tensions eventually resulting in the Mexican-American War.