If and when Russia and the US ally, it will be because Russia's seeking to unstraddle itself from the East-West faultline lest it be consumed in the dragon-fire of the Hegemon. Somewhat Eastern but somewhat Western, Russia's got a little of our idiocy and hubris at work, and they've steadily empowered and worked alongside China worldwide to their own peril.
Though Russia (or the "former Soviet Union's") militant atheists (or "communists") are indeed cunning and triangulate with ease the People of the Book. Holding out Israel as bait for the West, establishing the Jews as fulcrum, they've steadily turned Christian soldiers into relativist Peace & Justice pansies who look aghast on what they presume is the "militant faith" of Islam. Likewise, their consciousness-raising among Muslims has hardened them into heretics only too happy to kill in the name of God and they gaze in equal horror (and with righteous contempt) on the wholly decadent West.
(Sure, Islam's riddled with errors such that it's easy to turn ... but what we're confronting is actually communist repression and terror UNDER COVER OF ISLAM. Listen to Putin when he offers to exert his influence in certain "circles" in the Middle East. They've been organizing them for decades. What do you think Putin's talking about when he speaks of their "circles" of influence as he offers to influence Arabs even as he defends Iran's "anti-terrorist" fighting record!)
And what other terrorists did the Soviets sponsor? Israel's Zionists, of course.
What better than the messiah-less State of Israel (particularly an Israel to which the US was as Protector to ensure, as with the drip drip drip bombing of Iraq, consistent Anti-US feeling) to "organize" the disparate and wholly declined Muslim world? Piece of cake. Israel serves as the moveable fulcrum by which to "send a message" from time to time and it keeps not only radical Islam but faithful Muslims focused.
So ... we "Christians" fighting at the behest of a government who's mandated that all men ARE NO LONGER created equal, are in a world of hurt. The only cool thing is that Russia's hubris at outsmarting the West has left it somewhat lacking in keeping tabs on the East.
The truly cosmic conflict is not going to fall between this or that faction of the People of the Book ... regardless which bully of the material world's Celestial Economic Sphere back them. It's going to fall between East and West. That's why Russia is key and that's why I pray yet every day for the true conversion (bottom up, baby) conversion of Russia.
Here's some Chesterton to round out the Belloc for you: