The Mexican War of 1846–48 had ended with U.S. forces occupying Mexico City. Some in Congress proposed taking all of Mexico. From a military perspective, that was entirely feasible. But South Carolina senator John C. Calhoun, one of the nation’s prime defenders of slavery, objected. “We have never dreamt of incorporating into the Union any but the Caucasian race—the free white race,” he insisted on the Senate floor. “Are we to associate with ourselves, as equals, companions, and fellow-citizens, the Indians and mixed races of Mexico?”
Daniel Immerwahr in How to Hide an Empire