I think you highlight well the difference in opinion, on foreign policy, between libertarians/classical liberals in Europe and the United States. Alliances are sometimes a good option, and it pains me to see American libertarians dogmatically reject alliances in a spirit of reaction.
At the same time, European libertarians have yet to acknowledge a problem as old as Thucydides’ writings on the Delian League: that of free-riding. As NATO stands today, the European partners in the alliance (save for the UK and some newer, Eastern members) have been taking the US taxpayer for a ride.
This is a small injustice in the grand scheme of things, but it is an injustice nonetheless. The problem of alliances and free-riding extends far beyond NATO, of course. This is why I argue that alliances should be eschewed in favor of federations. I got this this idea from the likes of Ludwig von Mises, Adam Smith, and FA Hayek. The logic behind opting for federation over alliance runs something like this: if two or more countries can pledge mutual military aid to each other, but cannot abide forging closer economic and political ties, then the likelihood of each member of the alliance adhering to an agreed-upon charter is going to be very low.
Federation gets around this problem. Isolationism and empire do not.
Be sure to check out the back-and-forth between Edwin and General Magoon, too.