Lines of Russian troops in desert khaki greeted President Vladimir V. Putin in Syria in 2017. Declaring that Moscow had accomplished its mission in Syria’s civil war, Mr. Putin pledged that Russia was there to stay. “If the terrorists raise their heads again,” he said on the tarmac of a Russian air base, “we will deal unprecedented strikes unlike anything they have seen.” But over the last two weeks, as the rebels that Russia called terrorists swept across Syria aiming to topple one of Russia’s closest allies, President Bashar al-Assad, those “unprecedented” strikes were nowhere to be seen. Instead, with...