On March 8, 1846, General Taylor, under orders from Washington, moved his army toward the Rio Grande, and on the 28th of that month encamped on that river opposite the Mexican city of Matamoros.  He here erected a fort called Fort Brown, which commanded the city of Matamoros.  The Mexican troops near Matamoros were at the same time busily engaged in fortifying the city.  General Pedro de Ampudia, who commanded the Mexican forces at Matamoros, on April 12, 1846, addressed General Taylor a note requiring that within twenty-four hours he should retire from his position at Fort Brown and march beyond the Neuces, stating that the governments of Mexico and the United States were engaged in negotiations regarding the annexation of Texas, and that a failure or refusal of General Taylor to comply with this demand would be regarded by his Government as a declaration of war on the part of the United States.  General Taylor replied in substance that he was there with his army under orders of his Government, that he declined to retire beyond the Neuces, and that he stood ready to repel any attack which might be made upon him.  Soon after this correspondence General Mariano Arista was placed in the command formerly held by General Ampudia, and in May, with an army of six thousand men, he crossed the Rio Grande and attacked General Taylor at Palo Alto, and was signally defeated.  General Arista retreated on the next day to Resaca de la Palma, where he was again defeated and his army routed, and he retired across the Rio Grande.  General Taylor was now promoted to the rank of major general, and on May 18th took possession of Matamoros without opposition.

On September 9th he arrived at Monterey with about six thousand seven hundred men, chiefly volunteers.  General Ampudia held the command here with ten thousand regular Mexican troops.  General Taylor assaulted his position on September 19th, and after five days of almost continual fighting General Ampudia surrendered.  General Taylor then transferred his headquarters to Monterey, but guarded the city of Saltillo with a strong force.  He was about making an advance on San Luis Potosi, when a large portion of his force was ordered to join General Scott at Vera Cruz.

Concentrating his forces, some five thousand in number, he learned that General Antonio Lopez de Santa Anna was concentrating a force of twenty thousand men at San Luis Potosi, with a view to attack him.  On February 21, 1847, he took position at a mountain pass called Buena Vista, a few miles from Saltillo, where, being attacked the next day by the Mexican army under General Santa Anna, he defeated them, and Santa Anna retreated to San Luis Potosi.  This brief statement of the magnificent and almost unprecedented campaign of General Taylor is necessary to understand the part taken by General Scott in the war with Mexico.