Nile Gardiner, director of the Heritage Foundation’s Margaret Thatcher Center for Freedom, doubted Moscow would follow through on its threat. Russia, for instance, did not retaliate militarily after Turkey shot down its jet in 2015, he noted.Somebody please just shoot me. Last anyone heard, the Russians are in Syria with the permission of the Syrian government. If anyone is treating Syria as its own back yard it is the U.S. and its "coalition partners." They have no rights under international law to be in or over Syria without Syrian government permission, let alone to put troops on the ground and conduct military operations there.
He called the U.S. military’s decision to shoot down a Syrian jet a “welcome development.”
“Washington has sent a clear message to Moscow that it’s no longer business as usual,” he said. “For too long, the Russians have treated Syria as their own backyard.”
 "US signals involvement in Syria could escalate." By Rebecca Kheel, The Hill, 6/20/17.