Final answer:
Nicholas Trist ignored President Polk's recall from Mexico City to negotiate the Treaty of Guadalupe Hidalgo, which ended the war with Mexico and expanded U.S. territory, driven by beliefs in manifest destiny.
Step-by-step explanation:
U.S. diplomat Nicholas Trist, in defiance of President Polk's directive to return from Mexico City, seized an opportune moment to negotiate a peace treaty aligning with America's expansionist goals. Ignoring official orders, Trist orchestrated the Treaty of Guadalupe Hidalgo, ending the war with Mexico while significantly enlarging U.S. territory. The treaty added present-day California, Nevada, New Mexico, Utah, most of Arizona, and portions of Colorado and Wyoming to the United States.
Driven by the prevailing ethos of manifest destiny and a fervent desire to extend U.S. territory to the Pacific Ocean, Trist prioritized the broader national interest over immediate presidential instructions. His actions reflect the complex interplay of political, territorial, and ideological motivations that shaped American expansion during a transformative period in the mid-19th century.