We are talking about taxing the super rich in the EU to pay for a service (defence) that is currently paid by other people.

I agree with you that the economies of most EU countries cannot sustain US military spending levels.

My theory is that since the rich people in those EU countries are amassing wealth at a faster level than the economy is growing then we should use some of that growing wealth to ensure that the economy (and country) are properly defended. Also use the funds for education and infrastructure.