I don't understand how rigorous the training of the military is but if their body can take it, I don't see reason they should be denied to serve their country aside the issue of health because as a result of the medical conversion of their bodies, it might caused them some things to sustain them then they should be disallowed. However, its not necessary that they face combat so they can be deployed to other administrative units of the military instead of shutting them out.
If I am the one who will give orders, I will allow the transgender people to join the military force. Everyone can join the military as long as you are willing to serve and protect your country from all the terrorist that is relevant now in our world. Unlike the other people who join the military force but they are not doing anything to serve, they should be the one to get fired and lost their job. We should treat all people equal to prevent problems.