[1] After being invalided back to London for a time, Rees was placed in charge of a motor ambulance unit in Mesopotamia until 1919, when he demobilised with the rank of captain.
[2] Hugh Crichton-Miller invited Rees to work with him at a private psychiatric nursing home, Bowden House, Harrow on the Hill.
Rees married Mary Isobel Hemingway (10 September 1887 – 4 October 1954), the resident medical officer at Bowden House, in 1921.
Their marriage occurred shortly after Rees and Crichton-Miller created the Tavistock Square Clinic for Functional Nervous Disorders, a voluntary hospital which opened in 1920.
[3]During the war, Rees oversaw his colleagues' experiments with group psychotherapy, 'therapeutic communities', morale, rehabilitation, and selection tests.
Together with Henry Dicks, a fellow member of the Tavistock Clinic group, Rees was charged with the care of Hitler's Deputy at the secret prison locations where he was held following his capture after landing in Scotland.
[6] In 1945 Rees was a member of the three-man British panel (with Churchill's personal physician Lord Moran, and eminent neurologist Dr George Riddoch) which assessed Hess's capability to stand trial for war crimes.
This committee was chaired by Wilfred Bion, meeting twice a week to formulate a new way forward for their work at Tavistock, based on war-time experience.