Manifest Destiny. The U.S. provokes a war with Mexico, taking California and much of the Southwest. The United States is forged in wars and now stretches coast to coast. The next great flashpoint isn't about land, but slavery, with abolitionists fighting to stop its spread to the new territories, leading to the Civil War. Once over, the full power of the U.S. military turns to the western frontier where newly freed black cowboys and buffalo soldiers find purpose in the West. Meanwhile, the US government violently forces Native American tribes onto reservations.