Letterman proved the efficiency of his system at the Battle of Fredericksburg, in which the Army of the Potomac suffered 12,000 casualties, yet the command decisions of general officers in preparation for the later Battle of Gettysburg and during the Mine Run Campaign that followed that battle compromised Letterman's supply of medical equipment. His system was, though, adopted by the Army of the Potomac and other Union armies after the Battle of Fredericksburg and was eventually officially established as the procedure for intake and treatment of battlefield casualties for the entirety of the United States' armies by an Act of Congress in March 1864.