During World War II, Fairbairn and Sykes continued to refine their knife fighting techniques for military and paramilitary forces, teaching British Commandos, Special Operations Executive (SOE) personnel, selected American and foreign soldiers and covert espionage personnel, including members of the American Office of Strategic Services (OSS) and US/UK combined Operation Jedburgh teams.
Selama Perang Dunia II, Fairbairn dan Sykes terus pertajam teknik pertempuran pisau mereka untuk militer dan pasukan paramiliter, mengajar personil Pasukan Inggris, Special Operations Executive (SOE), personil Amerika dan tentara asing dan spionase rahasia terpilih, termasuk anggota dari American Office of Strategic Services (OSS) dan US/UK membentuk tim Operasi Jedburgh.