Ever read the fine print in a US passport?
Don't have mine with me but what does it say about "taking up arms" (fighting along side the enemy) against the US? If someone has it, please repro the verbage here. It is relevant.
I will vouch for that. I don't have my passport on me either, but I remember reading that you may lose your U.S. citizenship by serveral means, one of them being joining a foreign army.