April 30, 1861
GENERAL ORDERS, NO. 13.
The President directs that all officers of the Army, except those who have entered the service since the 1st instant, take and subscribe anew the oath of allegiance to the United States of America, as set forth in the tenth article of war.
Commanding officers will see to the prompt execution of this order, and report accordingly.
Abraham Lincoln, Executive Order Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/202289