[SideValve]

The USSR was founded because unbridled capitalism in their countries had led to terrible inequality and starving peasants being shot in the streets for protesting the lack of bread. WW1 didn't help with the view (shared by many German & British) that it was a largely political war being fought at the cost of commoners lives. They revolted in 1917. Its largely forgotten now that Germany revolted in 1918.
The progression from one system to another was expected to take many years and they never got there. Starting from a point of extreme poverty, the process of change created many production & supply problems which, together with political infighting caused as many deaths as the system it replaced.
Fast forward to WW2 they, even worse than us, were in no position to fight and struck a deal with Hitler. Once he'd gone as far as he could in Europe he set his sights on their huge natural resources. Had he succeeded he would have been able to concentrate of Middle East, Africa and, with Japan attacking our Far East empire, would doubtless had another pop at us. So, at a time when we were losing on all fronts, we had to bolster the Russians. For a large chunk of the middle of the war probably our most important role was our bombing campaign tying up German resources to give them a chance. The Russians sustained huge losses but also inflicted huge losses (an entire German Army surrendered after Stalingrad) and, in the end we, the Russians, the Americans and pretty much all our allies met in Berlin.
We were then very concerned (especially having portrayed them in a positive light through the war years) that Communism would further expand. The Cold War started and that was that.
Almost all countries carried on the progress towards mixed economies that had started after WW1 (at least partly to create fairer societies and thereby reduce the risk of revolution) but over the past 40 years "free-market" capitalism has been in the ascendancy again.