After securing the Rio Grande, General Taylor decided to move his army farther into Mexican territory. Monterey, the capitol of the Mexican state of Nuevo León, was his next target. General Ampudia moved the remains of his army there after the battle at Resaca de la Palma (Resaca de Guerro) and bolstered the city’s defenses. At the center of these defenses was the "Black Fort," an abandoned cathedral with 30 foot high solid stone walls and ample positions for cannon.

Taylor underestimated the Mexican resolve, and although outnumbered, he assaulted the city in September 1846. His losses were heavy and after three days, the fighting moved from house to house. Taylor agreed to let Ampudia disarm and withdraw from the city to prevent further bloodshed. They also negotiated an eight-week truce to regroup their battered armies.

This unauthorized truce did not sit well with President Polk, and Taylor was ordered to cancel it. Taylor’s increasing popularity was becoming a threat, and Polk decided to try to direct the war away from Taylor and his army. Santa Anna had other plans, though.