[191] 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.[192] 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.[193] Trist was authorized to make a payment of up to $30 million in exchange for these concessions.[193]
In August 1847, as he advanced towards Mexico City, Scott defeated Santa Anna at the Battle of Contreras and the Battle of Churubusco.[194] With the Americans at the gates of Mexico City, Trist negotiated with commissioners, but the Mexicans were willing to give up little.[195] Scott prepared to take Mexico City, which he did in mid-September.