He was apparently waiting for transportation promised him by the U.S. government, though his critics branded him inept. In July he moved his base up the Rio Grande to Camargo, but it was only in August that Taylor began planning the attack on Monterrey. By this time American strength on the Rio Grande had swollen to nearly 20,000 troops, nearly all volunteers. The principal military problem was logistical support of such a quickly expanded force. The Americans were susceptible to subtropical diseases and found it difficult to maintain sanitary conditions in the camps. Fevers, dysentery, and general debility were rampant, and the mortality rate from sickness was alarming. A determined Mexican attack in July or August would have proven disastrous to the Americans. The Mexicans did not attack though, because their central government had started collapsing. Rather than reuniting Mexico this war gave the Federalists an opportunity to rebel. Northern Mexico was a federalist stronghold, so as Taylor moved to the Rio Grande he increasing supported from the rebels. Soon Taylor began his advance Monterrey. He reached it on September 19, and began his attack on the morning of September 21. General William Worth soon joined Taylor, And within two days the much larger Mexican army began to retreat.

             The decisive campaign of the war was Scott's advance from Veracruz to Mexico City. Scott's expedition began at a staging area at the mouth of the Rio Grande in February 1847. He assembled an army of approximately 12,000, which was transported by sea to a beach about 3 mi south of Veracruz. Landing on March 10-11, it had surrounded the city by March 15. A combined naval and land attack began on March 22. Heavy shelling from navy guns forced the almost impregnable town to surrender on March 28. .

             During June and July, Santa Anna frantically prepared to defend Mexico City. On August 7, Scott began his advance from Puebla, following a route over lava beds and rough land to the south of Lake Chalco that Santa Anna had left relatively unprotected.