Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section I : Martial law -- Military jurisdiction -- Military necessity -- Retaliation - Art. 9.

Art. 9. The functions of Ambassadors, Ministers, or other diplomatic agents accredited by neutral powers to the hostile government, cease, so far as regards the displaced government; but the conquering or occupying power usually recognizes them as temporarily accredited to itself.