From that link: "In Wiborg v. U.S., 163 U.S. 632 (1985), the Supreme Court endorsed a lower court ruling that it was not a crime under U.S. law for an individual to go abroad for the purpose of enlisting in a foreign army."

So what is he actually supposed to have done that meant he faced a potential death sentence, and settled for a life in prison?

Courtw are supposed to be about justice - finding out if someone has actually broken a law, and terming what punishment is appropriate if it is proved that they have.