American settlers in northern Alta California captured Sonoma and declared an independent “California Republic,” an episode soon overtaken by U.S. military occupation during the broader war.
Representative David Wilmot introduced an amendment to bar slavery in any territory acquired from Mexico, bringing the status of slavery in the anticipated Mexican cession into national politics during the war.
U.S. and Californio leaders signed the capitulation at Campo de Cahuenga, largely ending armed conflict in Alta California months before the war’s formal end.
U.S. negotiator Nicholas Trist and Mexican commissioners signed the peace treaty, setting terms for ending the U.S.–Mexico War and ceding vast northern Mexican territory to the United States for compensation.
After debate, the U.S. Senate advised ratification of the Treaty of Guadalupe Hidalgo (with amendments), moving the agreement toward implementation and the formal transfer of the Mexican Cession.
The United States and Mexico exchanged ratifications, completing the treaty’s ratification process and clearing the way for the treaty to take effect.
The treaty was formally proclaimed, marking the official end of the war and establishing the framework for U.S. acquisition and governance of the ceded lands in the Southwest.
To implement the treaty’s boundary provisions, survey work and associated commissions began, initiating a multi-year effort to demarcate and map the new international boundary on the ground.
As part of the Compromise of 1850, President Millard Fillmore signed the act admitting California to the Union, incorporating a key portion of the Mexican Cession directly as a state.
The Compromise of 1850 included creating New Mexico Territory, providing a territorial government structure for much of the former Mexican Cession and shaping regional administration and land policy.
Also under the Compromise of 1850, Congress created Utah Territory, establishing federal territorial governance for a large interior portion of the Mexican Cession.
U.S. minister James Gadsden and Mexican officials signed a treaty for the purchase of a roughly 30,000-square-mile strip in present-day southern Arizona and New Mexico, intended in part to facilitate a southern rail route and to refine the border.
The U.S. Senate ratified the Gadsden Purchase treaty with modifications, advancing the final major adjustment to the contiguous U.S. border in the Southwest.
Mexican approval of the revised Gadsden Purchase treaty completed the bilateral process needed for the land transfer, after U.S. ratification and presidential signature.
With proclamations and implementation, the Gadsden Purchase became effective, finalizing the last major territorial acquisition in the contiguous United States and further clarifying the U.S.–Mexico boundary.
Mexican Cession and Incorporation of the Southwest (1846–1853)
