On February 2, 1848, representatives of the United States and Mexico signed the Treaty of Guadalupe Hidalgo, ending the war between those two countries that had begun two years earlier. This document ceded to the United States almost half of Mexico's national territory in exchange for a payment of fifteen million dollars. The discovery of gold in California a few weeks before the treaty was signed led to a massive western migration that changed the future of the country.

The Treaty of Guadalupe Hidalgo contained provisions promising to protect the civil rights of the more than one hundred thousand Mexicans who lived in the conquered territories. Their lands and ways of life, however, came under attack by the new settlers, and the treaty guarantees were largely ignored. This violation of the treaty fueled decades of conflict, especially over land. Furthermore, the acquisition of new...