The United States Military Is Not Currently Designed For Continuous Fighting.

 Why?

• Up until 2014, our defense contractors work building missiles in a way that maximized their profits but slowed down productivity.
• At best, we can sustain projectile-based fighting (offense and defense) for about a week. Also, a lot of the military budget is spent on Tricare. 
• Our stockpiles has been depleted (see also the above link) in other combat operations and replenishment is both costly and time-consuming (as in: would take years to replenish).  

All an opponent of the US Military has to do is survive the initial attack, kinda like: