The military should decide who is fit - and not fit - to serve.  If they decide trangenders are not fit (which is rational - higher rates of suicide, mental issues, emotional issues, etc.), so be it.

Even though he is our military's commander-in-chief, if Trump tweeted this without getting the support of our military leaders, shame on him.  But, if this is what the military wants - then good - in my view, trying to deal with social experiments is probably not good for combat readiness or morale.