Eurasia is a strategic prize without equal—which is why the world has been roiled, reshaped, and nearly destroyed by clashes over that supercontinent and the oceans around it. Since the early twentieth century, autocratic land powers, from Germany under Kaiser Wilhelm II to the Soviet Union, have sought to seize commanding positions in the world’s strategic heartland. And offshore sea powers, namely the United Kingdom and America, have sought to make the world safe for democracy by keeping Eurasia in balance. Now China and Russia lead a new axis of authoritarians that aims to create a radically revised international order. If they succeed, America and other democracies will be vulnerable and insecure. explains the revolutions in technology and warfare, and the rise of toxic ideologies of conquest, that made Eurasia the center of twentieth-century geopolitics—with pressing implications for the struggles that will define the twenty-first.