For almost three decades, Putin has been saying and many in Russia and the West have followed him in this that the Soviet Union came apart because the Bolsheviks set up union republics and gave them the right to leave, a right Moscow never expected them to claim but added in the hopes of eventually expanding the USSR to include other peoples as well. There is no doubt that that Leninist approach contributed to the way in which the USSR fell apart, but it does not explain fully why it did. That explanation lies in two other spheres, demography and...