John Wilkes Booth and company following Lincoln assassination. Treason was not specified I believe, but conspiracy was. The current imbroglio fits the same description without the presidential death.
There was some debate over the use of military tribunals, and AG James Speed prepared an opinion on the subject.
At the time of the assassination a civil war as flagrant, the city of Washington was defended by fortifications regularly and constantly manned, the principal police of the city was by Federal soldiers, the public offices and property in the city were all guarded by soldiers, and the President's House and person were, or should have been, under the guard of soldiers. Martial law had been declared in the District of Columbia, but the civil courts were open and held their regular sessions, and transacted business as in times of peace.Such being the facts, the question is one of great importance-- important, because it involves the constitutional guarantees thrown about the rights of the citizen, and because the security of the army and the government in time of war is involved; important, as it involves a seeming conflict between the laws of peace and of war.
Having given the question propounded the patient and earnest consideration its magnitude and importance require, I will proceed to give the reasons why I am of the opinion that the conspirators not only may but ought to be tried by a military tribunal.