There already is a considerable 'national defense' in America, is there not?
What good is defending a country that has no fuel to run its' business etc. etc. etc. (talking long term here, but that was not mentioned in the original post).
With a free source of clean energy, you would have no requirement for foreign oil. There would be no money going to your 'enemies' just because you like cheap gas & use a lot of it. They would not be able to buy more weapons and yours are already superior, so you could still shaft 'em if they tried anything funny. Your supply of wonderful clean & free energy would be in DEMAND all over the world. You would be able to SELL it to PAY for any type of national defense you desired.
Buy one get one free, or buy two and one's going to run out anyway.
Long term vs. short term
EDIT: you'd be able to research the cure for cancer with all the revenues too, so it's buy one get two free