Re-read the title you posted yourself......"Syrian Allies" aka, the Rebels!!! This isn't a US vs. Russia war. This mess is created and supported by the Jihadi lovers who don't want to show their face now, like KSA, and others.
The US had tried to provide support and training to fight the crazy ISIS terrorists. But if the government of Syria (apparently still in power), wants to invite the Russians, then we have no skin in the game anymore. Just don't cross the borders to the Western world. The Syrians really need to figure this out themselves!!!
The USAF actually will decimate the Russian air-force with much lesser flying power in a short amount of time, like a week, and close to Russia to be precise. But, Russia has the largest stockpile of flash lights. Why does the entire world has to suffer due to some idiot's imagination on PDF??
The Syrian government is still in power obviously and it invited the Russians. Now all parties involved have to deal with the consequences of this invitation
. The US / NATO need to protect allied borders and that's already being worked on.