On March 18, 1941, Brooker joined the SOE's training section, and trained scores of SOE operatives at Beaulieu on espionage and tradecraft prior to their departure to fight the Axis.[3][4]
In December 1941, Brooker was sent to Canada aboard the SS Pasteur with Lieutenant Colonel Roper Caldbeck to become the second in command of STS 103, or what is more commonly known today as Camp X.[3][5]
In August 1942, Brooker succeeded Roper Caldbeck as Commandant of Camp X.[3] He is said to have been the school's most popular commanding officer to this point.[6][7]
Bickham Sweet-Escott wrote:
"I have no doubt that OSS (Office of Strategic Services) got much more out of our training school in Canada, (Camp X) than from all the efforts of our party in Washington... What was unique about Oshawa was the personality of the commandant, Lieutenant-Colonel Brooker. Bill Brooker was a born salesman. He was a brilliant and convincing lecturer, and he had an immense wealth of stories from the real life of a secret agent to illustrate his points."
Brooker was noted as an unconventional instructor - he would occasionally interrupt his classes with exercises, including gunfire and other scenarios, and would ask his students to recall details like the number of shots fired and the caliber of the weapon.[7] He also gave them much sage advice: “If there’s anything loose in the intelligence business, you’re dead!” [8]
Brooker also trained OSS executives on the weekends on the fundamentals of training, creating "trained trainers" out of them.[9]
In the Winter of 1942, Brooker helped Garland H. Williams to completely redesign the training curriculum of the COI/OSS.[10]
Brooker also assisted, along with British Navy Commander N.G.A. Woolley, Millard Preston Goodfellow and William J. Donovan at the COI in creating an amphibious warfare training unit, that would become the OSS Maritime Unit training center.[11]
In March 1943, Brooker was reassigned to the OSS in Washington, D.C.[6][11]
Later in the year, William Donovan had Brooker deployed to Algeria to participate in joint SOE/OSS operations there.[10]
In 1944, Brooker would spend more time in Washington and New York, dividing his time in half between the two OSS operations, helping both to instruct students and build secret interception systems of German intelligences.[12]
