I mean, Stalin was planning to invade Hitler at the same time, and anti-communism was a basic pillar of Nazism.
There’s no necessary part of liberal democracy that demands that communism must be opposed through force. It’s not like Locke ever said “oh yeah, and if these Communist guys ever show up, kidnap them and throw them out of helicopters”.
Containment wasn’t a policy core to the ideology underpinning American democracy. It was possible (and I’d argue, preferable) to use other methods of defending American interests.
I mean, on an ideological level, Social Democracy as an ideology is entirely built around the idea of co-opting socialist policy goals to maintain a liberal democratic system.