What US Supreme Court ruling held military tribunals could not try civilians when civilian courts were open?

Ex parte Milligan, 71 US 2 (1866)

"Military commissions organized during the late civil war, in a State not invaded and not engaged in rebellion, in which the Federal courts were open, and in the proper and unobstructed exercise of their judicial functions, had no jurisdiction to try, convict, or sentence for any criminal offense, a citizen who was neither a resident of a rebellious State nor a prisoner of war, nor a person in the military or naval service. And Congress could not invest them with any such power."
Ex Parte Milligan case of 1866