In 1821 Mexico, having declared its independence from Spain, took control of the territory that now includes all of California, Arizona, New Mexico, and Texas, and parts of Nevada, Utah, and Colorado. But within twenty-five years, present-day Texas had been annexed by the United States, and at the end of the Mexican War the remaining areas were ceded to the United States under the Treaty of Guadalupe Hidalgo.
The treaty, signed in 1848, was not ratified by the Senate until 1850; the delay was caused by the unsuccessful efforts of Republicans to attach to the treaty the WILMOT PROVISO, banning SLAVERY in the newly acquired territory. For more than a decade an important constitutional issue was debated but not resolved: the question whether the treaty's provisions preserving Spanish or Mexican local law in the territory were themselves sufficient to abolish slavery.
The Treaty of Guadalupe Hidalgo gave all inhabitants of the affected territory the option of becoming United States citizens or of relocating within the new Mexican borders. Although some moved to Mexico, the overwhelming majority remained at home in what had become United States territory. As a result, for the first time in the nation's history United States CITIZENSHIP was conferred on people who were not citizens of any state. This action added fuel to a constitutional debate about the relation of national citizenship to state citizenship, a debate that continued until the FOURTEENTH AMENDMENT was ratified in 1868.
The international border remained unmarked and for most purposes unreal. Until 1894 there was no formal control over the border; United States IMMIGRATION statistics recorded the arrival of Mexicans only at...