Wikipedia is good for general knowlege. Legal charges are made with respect to a specific statute. In the U.S. treason will only ever be charged when we are at war.
There was a US Senator (John Breckinridge) who got expelled from the Senate for committing treason during the American Civil War. He did that by skipping out of the US Senate and joining the Confederate Army. He had quite a colorful career in the Confederacy while it lasted, fled to Europe after the war, and eventually returned to the US after the US gave a general amnesty to ex-Confederates in 1868. Wow.
https://en.wikipedia.org/wiki/John_C._Breckinridge