Hmmm................I remember something, what was it.................Oh yes!: WWII!
As has been explained, the only "communist" country that participated in WW2 was Russia. CHina turned communist in 1949, Although after the (1945) Korea had been partitioned.
A key point, however, that many people do not realize, is that as a result of the Soviet-Nazi " non aggression" pact (1938/39?) a deal was concocted in that when the Germans invaded Poland from the West, the Soviets moved in from the East to gain a " buffer" zone. That winter, (1939/1940) the Soviets invaded Finland in the " Winter War".
In that sense, Russia was at the "start" of WW2 as an aggressor, although it came to bite them in 1941 when they were invaded.