The US doesn't need to spend more on Defense, they need to redefine "Defense". 
Foreign policy & aid should be part of the overall defense strategy.

The battlefield is changing and how we engage the enemy, and with what, should determine the size and composition of the Armed Forces. 
Case in point, a modern day Arleigh-Burke class Destroyer has more capability (and probably lethality) than a WW2 Battleship.

There are ways to be stronger than simply adding more weapon systems & personnel to the arsenal