[153] In late 1845, He sent John Slidell to Mexico to purchase New Mexico and California for $30 million, as well as securing Mexico's agreement to a Rio Grande border.[155] Mexican opinion was hostile and President José Joaquín de Herrera  refused to receive Slidell.  Herrera soon was deposed by a military coup led by General Mariano Paredes,[156] a hard-liner who pledged to take back Texas.[157] Dispatches from Slidell warned Washington that war was near.[158]
Polk regarded the treatment of Slidell as an insult and an "ample cause of war", and he prepared to ask Congress to declare it.[159] Meanwhile, in late March, General Taylor had reached the Rio Grande, and his army camped across the river from Matamoros, Tamaulipas. In April, after Mexican general Pedro de Ampudia demanded that Taylor return to the Nueces River, Taylor began a blockade of Matamoros. A skirmish on the northern side of the Rio Grande on April 25 ended in the death or capture of dozens of American soldiers and became known as the Thornton Affair. 