Even though Europe was decimated by WW1, US and potentially Japan still had untapped military potential for full-scale intervention (basically they could do to Russia same thing British Empire did to China in XIX century).
Well the first red scar 1917-1920 wasn’t really big. The next red scare was from 1947 to 1960 and by that time I don’t think most countries would have the resources to invade the USSR and those that did wouldn’t want a 3rd world war