[AlexanderHowl]

Military prison is not something that anyone would want to experience because of the dishonorable discharge after finishing your sentence. I have known a few unfortunates that ran afoul of the system (some of them were old salts that were allowed back into service when they completed their time, others were given dishonorable discharges after they served their sentence). In the USA, companies that do business with the US government (most of them) are forbidden from hiring people with dishonorable discharges, and they cannot qualify for financial aid for school and a number of other government benefits.

When it comes to someone who is mentally unfit for trial, things become much more complicated though. Unless they are considered a threat to others, they will likely be held in a mental ward at a military hospital until they have been stabilized. If they are considered completely unfit for trial after being stabilized, I would think that most military prosecutors will not even attempt prosecution because they have better use of their time than losing a trial due to an insanity plea, so the service person would be medically separated and transferred to a civilian institution for the rest of their life. Alternatively, they will declare the issue a Personality Disorder and avoid giving them a medical discharge if they can get away with it (the US military is notorious about doing that, even though anyone with a Personality Disorder is supposed to be weeded out during Basic Training, as the service person is rarely in the state of mind to allow them to do anything about it).