I have always found the reaction to the Molotov-Ribbentrop Pact of Communists and Leftists all over the world (particularly in the USA) and then to the German invasion of the Soviet Union to be quite revealing.
They were completely, totally, 100% isolationalist, but as soon as the invasion happened, they turned into unrestrained advocates of full-on war.
No shame. No guilt. No explanation. Just a turn on a dime.
But you are correct...Nazis and Communists, two Leftist ideologies going hammer and tongs on each other...who could lose who wasn’t on one of those two sides?