Ok here is my more in depth analyse that i just pulled out of my ass.
If Russia is defending: Europe got no chance, Europe together with US got not chance too, i see a long bloody war, where NATO forces going home, thinking "wtf just happened, we were better everywhere...". Historically it just bad to try to conquer Russia, i am not saying it it impossible, and imo Germans would succeed if they were more friendly, but its just highly unlikely, so in any war where someone is trying to conquer Russia, my money is on Russia.
PS: Regarding Turkey, historically they got a bad record against Russia. They got like 2-7-2 record. But i don't know how good modern turkey is.
And no Mongols didn't conquer Russia, because it was no Russia at the time.
If Russia is attacking: Again historically, in offensive wars Russia were always a paper tiger. Look at what happened in wars with Finland and Sweden, yes Russia/Soviet won both but at what cost, so historically Russia did poorly against western nations- when Russia was attacking (If i remember correctly). And In offensive wars a good economy is important, something that Russia today lacks.
So i say they get stopped in Poland, or even Western Ukraine. :wink:
So yeah, i see a crushing defeat for Russia in an offensive war against Europe, if US is involve, i give it one month before Russia is totally crushed. The whole thing would just be suicidal.