Article III Section 3 of the Constitution reads:
Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.
The problem with your comment is that the government only has to accuse a citizen of fighting American forces. They don't have to prove it to anyone in any court or even in any document. The accused person has no rights at all under this law. In fact, they only have to say you were fighting American forces if they get caught holding you in detention.
It makes no difference if you're on American soil or not. You don't give up your rights as an American just by leaving American territory. Fighting American forces is treason, and the requirements for proving treason are clearly spelled out in the Constitution: testimony in OPEN COURT by two witnesses to the same overt act. Period.