The use of foreign aggression to rally support for the regime is not new in Russia. For many Russians, the idea that they were citizens of a powerful state was historically a vital part of their identity. The 2014 annexation of Crimea was presented as a return to Russian greatness and, in its wake, Mr. Putin’s approval rating rose to 80% from 60%. The annexation inspired a wave of euphoria and mass celebrations that lasted five years.