Why the heck and when was the United States considered reliable?

Reliable in what context?

Oh I see defensively reliable.

It might not make a lot of sense to overwhelmingly rely your national defense on a partner separated by an ocean.

I'm glad the EU is taking more responsibility for their own defense, and I'm also surprised to see so many European leaders acting surprised that they should have to, or the idea of a European defense as a novel idea.