The answer is fairly simple with two basic ways to interpret it I would think. One is to regard the meaning of ‘powerful’ as a physics and structure equation, In my opinion U.S. military power tends to evolve with advancing weapons designs and therefore during J.F.K.’s administration there were more nuclear weapons and better technology than before.
The second way to regard powerful is the overall effectiveness of the force. In that case I would say that Dwight Eisenhower had a more powerful military because of his leadership. An example can be found in the a video of the Songhai empire of Africa and its history, leadership etc. They had a couple of great rulers and one that was quite wicked slaughtering people without need because he seemed to be inclined to do so and didn’t differentiate between civilians and military. One general and reformer with a some wisdom named Askia Muhammed rose to power and defeated a larger army in battle once simply because he was a more experienced leader.