The U.S. changed the course of the war with its invasion of Mexico's heartland through Veracruz and ultimately the capture of Mexico City, following hard fighting. In March 1847, Scott landed in Veracruz, and quickly won control of the city. The Mexicans expected that yellow fever and other tropical diseases would weaken the U.S. forces. With the capture of Veracruz, Polk dispatched Nicholas Trist, Buchanan's chief clerk, to accompany Scott's army and negotiate a peace treaty with Mexican leaders. Trist was instructed to seek the cession of Alta California, New Mexico, and Baja California, recognition of the Rio Grande as the southern border of Texas, and U.S. access across the Isthmus of Tehuantepec. Trist was authorized to make a payment of up to $30 million in exchange for these concessions.