First, if you think Stalin wouldn't have taken his armies right to the Atlantic coast of France after crushing the Nazis, you'd be badly mistaken. It was specifically U.S. military power (and the atomic bomb) that stopped him from trying this. The rest of the allied powers were too exhausted and small to be credible deterrents.

After that, over the following decades on multiple occasions, the same thing applied, a major power, like the USSR or China was tacitly or openly stopped from taking things further or attacking more specifically because of US presence in the background. This applies to South Korea for sure, but also to many places in central Europe and elsewhere.

Today, the U.S. remains as the biggest deterrent of further Russian encroachment into places aside from those that it specifically considers its rightful domains (Ukraine, for example, where even foreign threats of retaliation couldn't stop a nationalistic sense of having the right to attack). I'd argue that this is one of the major reasons why Finland wasn't invaded long ago. (U.S. presence and the dawning realization among the Russian leadership that its supposedly strong remodernized military is much shittier, weaker and more corrupt than it was "supposed" to be.)