Those groups begin with women and gays, and continue with Muslims, atheists, and ends God knows where.
Please explain why women, gay people, Muslims, atheists, etc can't be good military personnel. I know this much: The soldiers I've talked to and seen polled about it overwhelmingly either support or don't care about these kinds of measures. This might have something to do with the fact that when you're in a firefight you care more about whether the rest of your unit are good shots than who they like to kiss or what they think about spirituality.
Now consider that beards have been outlawed by our military for decades, based on "discipline" considerations. No redneck, no Jew, no mountain man has been permitted to display a beard while in uniform.
Please explain why wearing a beard displays a lack of discipline or lack of military readiness. I'm really not understanding what the purpose of that kind of rule could possibly be, except some silly holdover from the 1950's that stereotyped bearded men as drunkards and foreigners. During the Civil War, wearing a beard was very common, and it doesn't seem to have had any effect on the skill or bravery or readiness on the troops (or at least not enough that anyone made any mention of it whatsoever in any military documents).
While a liberal or a progressive may feel that to be a "good thing", the fact is, our military is being improperly used to advance a number of political agendas.
Well, let me tell you of another time the military was used to "advance a political agenda": Racial integration. In 1948, Harry Truman issued an executive order desegregating the US military. Today, black people are more likely to join the military than white people, in large part because they know that the organization will treat them fairly and give them a good chance of a career. We'd probably lose 5-10% of our military personnel had Truman not done that.
We no longer have the military that we had thirty or forty years ago.
No, we don't, and we're at the very least no worse off for it. Running down the list of US military operations between 1974 and 1984 (the "glory days" you seem to be yearning for), the most significant military actions were the evacuation of Vietnam and the invasion of Grenada. Do you really think those were more difficult military operations than the invasions of Iraq and Afghanistan?