Lately I was reading the Invasion America series by Vaughn Heppner (https://www.goodreads.com/series/106530-invasion-america) and it left me with this idea that it's before a great army is needed that it can be built and organized. In those books a natural disaster left many countries with food shortage so they attack America who happens to have a lot of intact farmland, and the army is not big enough to protect it.
I think the problem at the moment is not the size of the army; it's the lack of innovation in its methods and policies. Basically the army is still following principles enacted in the 1800s by Von Clausewitz; this makes the army a great invasion force but very clumsy in every other aspect of warfare.
And innovation does not mean selling tanks to buy supercomputers for the NSA. It means figuring out better ways to protect the country in the physical and virtual worlds.