The day that Russia invaded Ukraine, Feb. 24, the legacy of Mikhail S. Gorbachev loomed over President Vladimir V. Putin’s predawn speech. “The paralysis of power and will is the first step toward complete degradation and oblivion,” Mr. Putin intoned, referring to the Soviet Union’s collapse. “We lost confidence for only one moment, but it was enough to disrupt the balance of forces in the world.” For Mr. Putin, the end of the Soviet Union was the “greatest geopolitical catastrophe of the century,” a “genuine tragedy” for millions of Russians because it left them scattered across newly formed national borders....