>>509544370 (OP)They did—and still do—but shared threat overrides rivalry.
Communists and capitalists teamed up to defeat the Nazis because fascism threatened both systems, albeit for different reasons:
• For capitalist democracies (like the UK and USA), Nazism was an existential military threat and a challenge to liberal global order. *spits*
• For the Soviet Union, it was a genocidal invasion aiming to enslave or exterminate entire populations and destroy socialism in Europe.
Temporary alliances are common in geopolitics when survival is at stake. After the war, the rivalry resumed—sharper than ever—with the Cold War. But in 1941–1945, necessity created cooperation, not friendship.