Impersonator

 https://en.wikipedia.org/wiki/Ferdinand_Waldo_Demara

 The following year, Demara began his new lives by borrowing the name of Anthony Ignolia, an army buddy, and going AWOL. After two more attempts in monasteries, he joined the Navy where he trained as a hospital corpsman.^[5]:80 He did not reach the position he wanted, faked his suicide and borrowed another name, Robert Linton French, and became a religion-oriented psychologist, who taught psychology at Gannon College (now a university) in Erie, Pennsylvania. Afterwards, Demara served as an orderly in a Los Angeles sanitarium, and served as an instructor in St. Martin's College (now a university) in the state of Washington. The FBI captured him, and he served 18 months at the Naval Disciplinary Barracks, San Pedro, California, for desertion.^[4]

After his release he assumed a fake identity and studied law at night at Northeastern University, then joined the Brothers of Christian Instruction in Maine, a Roman Catholic order.^[4]

While at the Brothers of Christian Instruction, he became acquainted with a young Canadian doctor named Joseph C. Cyr.^[4] That led to his most famous exploit, in which he masqueraded as Cyr, working as a trauma surgeon aboard HMCS Cayuga, a Royal Canadian Navy destroyer, during the Korean War. He managed to improvise successful major surgeries and fend off infection with generous amounts of penicillin. His most notable surgical practices were performed on some sixteen Korean combat casualties who were loaded onto the Cayuga. All eyes turned to Demara, the only "surgeon" on board, as it became obvious that several of the casualties would require major surgery or certainly die. After ordering personnel to transport these variously injured patients into the ship's operating room and prep them for surgery, Demara disappeared to his room with a textbook on general surgery and proceeded to speed-read the various surgeries he was now forced to perform, including major chest surgery. None of the casualties died as a result of Demara's surgeries. Apparently, the removal of a bullet from a wounded man ended up in Canadian newspapers. One person reading the reports was the mother of the real Joseph Cyr; her son at the time of "his" service in Korea was actually practicing medicine in Grand Falls, New Brunswick. When news of the impostor reached the Cayuga, still on duty off Korea, Captain James Plomer at first refused to believe Demara was not a doctor (and not Joseph Cyr). However, faced with the embarrassment of having allowed an impostor into the navy's ranks, Canadian officials chose not to press charges. Instead, Demara was quietly dismissed from the Royal Canadian Navy and forced to return to the United States. The MASH episode "Dear Dad... Again" included a one time character Captain Adam Casey, likely inspired by Demara's exploits, who performs several surgeries, but turns out not to be a real doctor.