Military and political analysts said winning the war in Ukraine has become an all-encompassing goal for Mr. Putin. That outcome, they said, would justify to the Russian leader the conflict’s tremendous human and economic losses, safeguard Russia’s statehood and global stature and compensate for strategic failures elsewhere, such as in Syria.

Assad fled to Moscow as opposition forces advance towards Damascus. Sources say Russian intelligence agents organized the escape, flying Assad out of a base in Syria before the aircraft's transponder was turned off to avoid being tracked.