Anyone who thinks military promotion - at least in peacetime, and at least above a certain level - don’t have substantial political elements is extremely naive.
Everything I’ve ever read or heard about the subject demonstrates that virtually every appointment above Army/Marine/Air Force Lieutenant Colonel or Navy Commander has a political element. Meaning not just a “service politics - plays well with others” element, but an actual political element. While it’s not dominant at the Colonel/Navy Captain leve, it’s in play, and once there are stars in the equation it becomes increasing important as the number increases.
Think about the way that over a hundred general officers retired within a year or so after the Clintons took over in the early 1990s. Most of the general officers we fought the 2nd Gulf and Afghanistan wars with got their first stars under the Clintons. Even the “beloved” Petraeus, who is a liberal - any conservatives who thought he’d be a good candidate would have been profoundly disappointed in him.
And, now, most of the general officers who got stars under Bush II are out or almost out and almost all of the general officers, and the rising LTCs and Colonels/Commanders and Captains are all those who’ve spent most of their careers under Democrats. You think the officer corps hasn’t become much more liberal over the past 20 years?