You can have your status as a resident alien revoked in certain cases of felony convictions. It's more than likely most of these folks fall under that case. In order to serve or be drafted you need to have resident alien status (green card). This can be revoked as mentioned before regardless of prior service. Serving does not guarantee citizenship but can significantly move you along in the process. Happened to my dad and his brothers in the 60s-70s and several guys I served with who processed and got their citizenship while in service. The military WILL NOT put in paperwork for you, you have to notify your command and begin the process yourself through admin.