Why did Mexico give up the land included in Mexican cession?

The land that was included in the Mexican Cession was not relinquished voluntarily. Mexico was defeated by the United States in the Mexican-American War, which concluded in 1848. As part of the peace settlement, Mexico was compelled to cede significant territory to the United States, including present-day California and much of the Southwest.

The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, formally transferred sovereignty over the Mexican Cession to the United States. The United States paid Mexico $15 million as compensation for the lost territory, and the United States also agreed to assume responsibility for the debts of Mexican citizens who lived in the ceded territories.