During his service with the International Police in Shanghai, Fairbairn reportedly engaged in hundreds of street fights in the course of his duties during a twenty-year career.

He also developed numerous firearms training courses and items of police equipment, including a special metal-lined bulletproof vest designed to stop high-velocity bullets from the 7.63x25mm Mauser pistol.

2 Dutch Troop 10th Inter-Allied Commando forces, US Rangers candidates, and personnel of the Office of Strategic Services (OSS), mainly for close-combat, pistol-shooting and knife-fighting techniques.

In 1941, he appeared in a training movie titled "Unarmed Combat", demonstrating many of the strikes, holds and throws of Defendu, for use by the Home Guard, Commandos, and military services.

For his achievements in training OSS personnel, Fairbairn was promoted eventually to the rank of Lieutenant-Colonel by the end of the war, and received the U.S. Legion of Merit (Officer grade) at the specific request of OSS-founder "Wild Bill" Donovan.

He described this system as based primarily on his personal experience, which according to police records included some 600 non-training fights, by his retirement at age 55 from the job of Assistant Commissioner in 1940.