Earlier quoted context omitted.
The last time the US followed an isolationist foreign policy, the Europeans got themselves into two world wars in a row. It's true that Europe has a continent-wide mutual defense agreement now. What you didn't mention is that it's called NATO and the US is the backbone of it. That's part of how it works. Otherwise you don't get the combined forces of Europe, you get indifference at best and European war at worst. Loo…
Your information appears to be a generation out of date. The EU in particular have been working on collective defence agreements independent of NATO for quite some time. While it's true that if current US foreign policy went away overnight it would probably make certain other parts of the world less stable for a while, such rapid change couldn't actually happen as a practical matter of international diplomacy and eve…
That's not a coincidence, it's a benefit provided by the US military's overwhelming superiority.
The US military has also taken an extreme share of the burden of keeping global trade / shipping lanes open and safe for operation - and it has done an extraordinary job at it.
The USSR wasn't going to just stop mid way through Germany, and Russia was obviously not going to just stop at Georgia. Who is going to act as a big enough deterrent to them? The same countries that prevented genocide in Europe's backyard in Kosovo? No, only the US is a powerful enough threat to Russia to keep them from going on a non-stop annexation spree.