Comment by pfdietz

Strategic motivation? If one assumes the US is going to be globally involved, yes, but that's begging the question.

Economic motivation? Not so much now, with the US being a dominant oil producer, and with petroleum itself losing importance. Even then, it's questionable if this could justify the full cost of the US military.

I think the original motivation was two fold: it was a combination of some sort of moral obligation to defend the "free world" from authoritarians, and (after WW2) a desire to keep small countries (and recent WW2 enemies) from deciding their only option for defense was their own nuclear deterrent.