In 1941, promoted to lieutenant colonel, Foster assumed command of 4th Reconnaissance Battalion (4th Princess Louise Dragoon Guards), the recently activated scout formation assigned to 1st Canadian Infantry Division in England.

Later, swapping commands with Chris Vokes (because Vokes had a poor relationship with new I Canadian Corps commander Charles Foulkes),[5] he led the 1st Canadian Infantry Division in Italy, then returned with this division to North-West Europe as part of Operation Goldflake.

When asked by his son (author Tony Foster) why the death sentence had been imposed he replied, "Because I had no choice according to those rules of warfare dreamt up by a bunch of bloody barrack-room lawyers who had never heard a shot fired in anger.

Upon retirement in 1952 he took the civilian appointment of chief administrator of the Central European District, Imperial War Graves Commission.

In 1959, he married his third wife Mona Leonhart (née Parsons), a Canadian spy for the Dutch Resistance during the Second World War, and was appointed honorary aide-de-camp to Governor General Georges Vanier.

Tony Foster wrote a biography of his father's life, with particular emphasis on his interactions with Kurt Meyer, both on the battlefield and in the courtroom.

[12] The Foster family donated Harry's military uniforms, medals, battle maps and other historical items of interest to the Army Museum of the Halifax Citadel where they are part of the WWII display.