Seated on the same side of an antique table before a large audience, Presidents Barack Obama and Dimitri Medvedev smile at one another as they exchange documents encased in red and black leather folders. The new START treaty has been signed, completing a journey Obama began when he visited the Czech Republic a year ago and announced his intention to begin ridding the world of nuclear weapons. He has reset relations between Russia and the United States and fulfilled a dream of another American President: Ronald Reagan. The cold war is over. Back in America, the former half-term governor of...