Major wars unleashed new and unexpected forces, the authors show, and in post-war periods policy-makers were faced not only with the reappearance of old power-political issues but also with quite unforeseen challenges. In 1918, a hundred-year-old order based on a balance of power among the states of Europe collapsed, leaving European and American leaders to deal with social, ideological, and ethnic crises. After World War II, hopeful plans for peace were checked by nuclear rivalry, international economic competition, and colonial issues. And unexpected challenges after the Cold War -- global economic instability, ethnic conflict, environmental crises -- joined wi