Ukraine is just the beginning of a EU-wide civil war due to their hollowed-out economies. The US will destroy that which it feels it can no longer control (like collapsing vassals), so as to keep Russia from benefiting as the successor empire. Russia naturally wants to stall for time as the ability of the USA to cause trouble of that nature is quickly fading and Russia's star is quickly rising.

Which is pretty much the situation causing WWI between England and Germany. The difference is, this time there's no vast untapped Anglo well of strength to pull on -- Most of the 3rd world wants to be neutral or are with the Russians.