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.