> I've seen a lot of very proud Europeans claiming the US has had no stake in defending them
And I see a lot of proud Americans who state that they save the world out of pure altruism.
I have breaking news for you: the "extraordinarily expensive reserve capacity to wage warfare" is here because it serves USA's interests, and no one else. I highly doubt that the military policies of your government are conceived with the interests of us Europeans in mind (although there is overlap between ours and yours). I'm pretty sure that, except for a few mistakes, it has been immensely economically profitable for the USA overall, allowing you to influence or overthrow governments, ensure a hand on strategic resources as well as allow you to export your goods and culture pretty much everywhere through strategic alliances (I'm thinking especially about Japan and South Korea). That's the reason why there are USA bases in Asia, Europe, the Middle East and everywhere else.
By the way, as a whole Europe is the 2nd military spender, with France and UK matching Russia's spendings, and when added together also matching China's spendings (despite having less than 200 times the population).