Actually, by serving in "the armed services of a foreign state," Lindh may already have lost his US citizenship. Other ways are, formally renouncing US citizenship before a US consular officer overseas and surrendering one's US passport, serving in the government of a foreign country, making an oath or declaration to a foreign state, being naturalized in a foreign state.
It would be up to the State Department to decide whether JWL still retains US citizenship. If he does not, he can't really be tried for treason, only deported to Afghanistan possibly on grounds of being an "undesirable alien."