The President has made a lot of noise about how he thinks our allies are cheating us by paying less than their "fair share" for NATO and other defense alliances. But the fact is that we're the 800-lb. gorilla in the room: As the world's only superpower, we benefit a whole lot more from a peaceful world than anybody else, and we would be harmed a whole lot more if that peace went away. Even if we had no allies at all, we would still be forced to do about the same amount of heavy lifting as we do today -- maybe even more. That's because national defense is subject to what's called the free-rider problem.
Imagine, if you will, that there were no national air forces -- just private ones. If Bill Gates had his own private air force, it's safe to assume that most everybody else in Seattle could get by under the umbrella of his protection. After all, you couldn't really protect Redmond without protecting the rest of the nearby area. While it's obviously silly to imagine people having private air forces, this extreme example provides a good illustration of the free-rider problem. If one dominating force has a reason to do something, then it's easy for other parties (who might want the same thing) to skirt by without having to pay for it.
The best thing for Bill Gates (in this parallel universe) would be to get as many people as possible to commit to going along with him to help in paying for his local air force. They probably wouldn't end up paying as much as he'd like, and they'd probably pay a lot less (proportional to their income or wealth) than he would. That, unfortunately, is just the way things go. He can't spend nothing, so he has to spend a lot. His best hope would be to encourage his neighbors to pitch in as much as they can.
And so it is with us and our allies in the world: We can't spend nothing on defense. And even if our allies were to spend nothing at all, we would still spend an incredible amount. So the best we can do is encourage them to chip in -- while recognizing that we can't really protect ourselves without also protecting them, too. Maybe it's not ideal, but it is reality. And it's unavoidable.