There were two big puzzles confronting structural theories of international relations at the beginning of the 1990s. The first was straightforward: why had everyone been surprised by the dissolution of the Soviet Union? The USSR had been the second pole of a bipolar world order, and theories of world politics should probably be able to account for the advent, and the exit, of the superpowers that shape the world those theories purport to explain. The second was more vexing – and more