He’s gone through three defense secretaries in six years. That tells you quite a bit about the situation. Bush has one through six years in a much more tumultuous time.
As one of the pundits observed, when George W. Bush made a national security decision, debate within the staff stopped and everyone fell in line. With Zero, the decisions are made by a select few at the White House—who have little expertise or competence in the field—and inputs from the Pentagon are not welcome. When you push back a little too hard, you find yourself next to the POTUS, who is announcing your retirement.
The most likely candidates to replace Hagel (Michelle Flournoy and Ashton Carter) are unimpressive. Both are Democrat policy hacks who worked their way up the food chain. Neither served in uniform, but they will certainly toe the administration line, while wasting time (and resources) on various PC issues.