US spends more on military than every Euro country combined. If they are attacked we have to defend them and they know it. They would contribute a relatively insignificant amount if we were ever attacked.
If? In the entire history of NATO, the mutual defense provisions of the treaty have been invoked exactly once, and that in response to an attack on the US, and the non-US NATO contributions were not insignificant.
that's the point
Many Europeans gave their lives in Afghanistan, responding to an attack on the U.S., and also in Iraq on a fools errand. The U.S. is the predominant power, but that doesn't make others insignificant.