I have read through most but not all comments as the threads tend to get off topic after a bit, so I apologize if I am duplicating some input on this subject. Many here have postulated that banning transgenders from the military is a matter of mental health concerns or military budget concerns. Truthfully, it is neither of these.

Its a readiness concern and I'll explain why. Under the Obama era policy, the servicemember had a number of options once they were clinically diagnosed with gender dysphoria. 1. They could seek mental health counseling to deal with issues like depression or social stigma. 2. They could seek hormonal therapy but not undergo sex reassignment surgery. 3. All of the above plus undergoing surgery.

While all three of these cost money and resources, the estimated costs are irrelevant to the DoD medical and personnel budgets. I have spent more procuring a single "new" (read refurbished) 1970's era circuit card from Lockheed Martin than the entire estimated cost of the transgender policy.

As far as mental health goes, its also irrelevant. Gender dysphoric people will join and serve the military whether they can do it openly or not. Having servicemembers with this condition doesn't magically cure them or make them go away because there is now a policy in place. They will simply suffer in silence or act out similarly to Private Manning.
Furthermore, people in and out of the military have mental health issues that are treated daily and continue living functional and fulfilling lives. The DoD already has a mental health infrastructure in place solely to treat or separate military personnel. Servicemembers, just like non-military civilians, already serve with mental health issues (to include gender dysphoria), this new policy will simply prevent them from getting help for it.

The key issue is the time spent away from the unit/squadron/ship while the servicemember undergoes their required medical treatment. The military, regardless of branch, is composed of small cells of specialists. Generally no more than 2-3 people in a unit are trained to complete the same job, though this varies depending on the unit's function and size. When somebody leaves for 6-12 months to receive treatment, there isn't an endless pool of people waiting around in the system to replace that person. And if there were, the servicemember doesn't get removed from the unit's roster and won't be replaced anyways. To the manning folks, on paper it looks like that person is still with the unit and fulfilling their function.

So, its not like a corporate system where so-and-so leaves, the manager calls a temp agency, and a new body shows up to fill that job for 6-12 months. That person is simply gone. So now the 2-3 specialists turn into 1-2 specialists IF the unit was already fully manned which they likely aren't. So you may be left with 1 person, maybe 2 to do the job of 3 servicemembers and that simply is not sustainable over the course of a deployment. The unit is no longer capable of performing whichever maintenance or operational function those specialists were performing, which then makes the unit and military less capable to fight and win wars.

By and large, many servicemembers don't care whether somebody is transgender. We are close knit and supportive of each other due to the way in which we all work, sleep, and train together for months on end. However, when it comes to losing readiness and suddenly one maintainer or operator becomes responsible for doing the work of three people, we realize the situation could become unsafe for everybody and suddenly the personal needs of one person don't overcome the safety of everybody.

What should happen and what should be the policy is that those diagnosed with gender dysphoria, by a military psychologist, should be evaluated for an appropriate level of treatment on a case by case basis. Those who can be treated without being removed from a unit and without hurting readiness should be treated. Those who require more in depth treatment like hormone therapy and sex reassignment should be separated from the military since they are treatment options which are mutually exclusive with unit readiness. But note this approach is a case-by-case professional evaluation of an individual's condition and not a blanket statement/tweet policy approach that is inherently discriminatory in principle.

EDIT 1: I realized I didn't address the mental health argument, so I added a bit about that.
EDIT 2: I also realized I didn't propose an alternate policy solution, so I added a bit about that at the end which hopefully proposes some solutions vice noting all of the problems with the military-transgender situation.