Of course everyone in the U.S. military functions as a US citizen subject to the Uniform Code of Military Justice and all the code the legislature can compile. After discharge from any of the services none of these ex-servicemen are required to obey either the UCMJ or any legislated code. The status of 'citizen' is one of office. This office is also known as 'trustee' or 'fiduciary' to the public trust. So what do you do? Truth and accuracy are important to any question of citizenship. If you have an office then state it. If you have no office then you are not a citizen BUT you are still ELIGIBLE to be a citizen. Being eligible is not the same status as actually holding an office of public trust. Being eligible does not make you SUBJECT. The eligibility to an office of public trust stems from one or any of these preconditions:
JUS SOLI ... you were born on U.S. soil (in the case of a U.S. citizen) or on State soil (in the case of a state citizen). You might also have been born on soil where a treaty exists with a foreign entity (France, Spain or Mexico for example). The Civil Law as practiced by these countries recognize your nationality (not citizenship) if the land you were born on was once part of their territory.
JUS SANGUINIS ... your blood was a citizen (U.S., State or other nationality). This means there is a paper from some court that established that your newly arrived grandfather from Germany (or wherever) went in front of a court and established that he was a naturalized citizen.
Old law from Pennsylvania established that anyone was considered a citizen who paid Pennsylvania taxes. I believe I would hesitate to pay a tax bill under these conditions. Quite possibly this rule is considered the case in all of the several States as well.