One hundred and seventy years ago, on February 2, 1848, the Treaty of Guadalupe Hidalgo expanded the territory of the United States by over 500,000 square miles, not only making it inevitable that we would become a Pacific power, but setting the stage for what may be the most complex border relationship between any two nations. The treaty formally ended our War with Mexico, but accelerated our headlong plunge toward the Civil War, intensifying the debate over the geographical expansion of slavery into our newly acquired territories.