The U.S. - Mexico War (1846-1848) is the largest and most significant conflict ever fought between two nations in the western hemisphere. Yet it remains an understudied and underappreciated event for both countries. In Mexico, the war left a legacy of bitterness that can be seen to this day. In the United States, which emerged from the war as a continental empire, tensions between North and South soon eclipsed the conflict in the national imagination. Abraham Lincoln alluded to these tensions when he described the United States in 1858 as "a house divided." A decade earlier, however, North America was a divided continent, the site of conflict between two republics that had forged two very different destinies since winning their independence from European powers.