You overestimate Russia's capabilities. Its economy is crumbling and the so called huge fleet of tanks is mostly standing in military vehicle graveyards and will need a lot of money spent to even restore them at all. Russia would not be able to have long supply lines especially when they get sabotaged and their cavalry would become useless when they arrived around Carpathians and the forests of central europe. European AA is superior to Russian AA which doesn't have the advantage of position. If the war gets dragged on for a while, it gives time to major powers to make an economy of war (Germany, UK, France, Northern Italy). Populations would start gettijg drafted to support the professional armies even if their equipment would be slightly old at first. There is no way Russia can overwhelm Europe over mountains, and rivers/forests of central europe. It vehicles useless, defenders would be entrenched and use guerilla warfare, conquered local populations (Poland for example) would keep sabotaging and harassing the Russian army behind its lines (Poland for example has huge civilian militias already). Russia would have to spread their manpower to maintain the conquered lands and maintain supply line and comms and it would lack air superiority. The only advantage of Russia is their stock of oil but western europe can adapt their ports to receive oil in other ways. Considering Russia's economy, it wouldn't be long until the people starve and rise and a part of the army has to be sent back home as potential rebellions would be financed by the west to divide Russia. Morale of troops and civilians would be higher in the west because it is a defensive war, defending home is one huge motivation and considering Russia would stagnate, it would look even better. Russia on the other hand would fight an imperialistic and costly war while their life quality gets even worse and the soldiers would feel the same. I don't see how Russia can win, their army is mostly filled with obsolete equipment, afterall their military budget isn't even that high so there is definitely a problem with what they say they have. Europe alone has enough nukes to assure destruction of all Russia, which prevents Russia from going nuclear to hit strategic positions. I think people underestimate Europe and overestimate Russia. If Europe had to rely only on themselves, their military could surpass the US military if they wanted to because the EU economy is similar to the US so such a budget could definitely be possible if needed, but it would affect quality of life (possibly) to become similar to the US aswell. No matter what, there is a reason the EU is called an ascending superpower. I would even say it is not in the US' interests to leave Europe alone because our military would have to get pumped to a point we would be more than a match for them (we already are on homesoil).