Stop for a moment and ask not simply why the Soviet Union broke up, but why it fractured into 15 pieces, rather than many more, corresponding, say, to the country's vast number of ethnic, or religious, or linguistic groups. Walker's answer relies on the concept of sovereignty, and specifically on the peculiar institutional and normative force flowing from "ethnic federalism." He reminds the reader with skillfully reconstructed detail of the intricate and delicate emendations of "sovereignty" already under way in Moscow and the republics before the collapse. He also notes that the only postsocialist regimes to come apart at the seams were "ethnic federations" -- the Soviet Union, Czechoslovakia, and Yugoslavia. Walker is not suggesting that federalism caused the collapse of the Soviet Union; only that it provided leverage for those who wanted independence, a "supply-source" of independence for those who did not, and a framework within which the outside world could manage the unexpected.