This is one of the reasons why leftists are pretty much universally for massive cuts in military spending. Which makes sense - you can't have both.
The Europeans would have to cut their own social programs if they wanted to do any peacekeeping / humanitarian aid operations - all of that is contingent on American logistical support. Hell, my guess is that they would castigate a more leftist, benign American government; it would make things a hell of a lot harder for a lot of people.