The Fairburn-Sykes fighting knife.

 
Shortly before World War II William Fairburn and Eric Sykes designed this weapon, the Fairburn-Sykes fighting knife.

The knife was used by the British commandos ( including the just created S.A.S ) in WWII  but became popular with many other special forces units as the war progressed. The knife also saw service in Korea and Vietnam. This is from Wikipedia.

The length of the blade [ 7 inches]was chosen to give several inches of blade to penetrate the body after passing through the three inches of the thickest clothing that was anticipated to be worn in the war, namely that of Soviet greatcoats. Later production runs of the FS fighting knife have a blade length that is about 7.5 inches.

The F-S Fighting Knife was designed exclusively for surprise attack and fighting, with a slender blade that can easily penetrate a ribcage. The vase handle grants precise grip, and the double-edged blade is integral to its design. Fairbairn's rationale is in his book Get Tough! (1942).

"In close-quarters fighting there is no more deadly weapon than the knife. In choosing a knife there are two important factors to bear in mind: balance and keenness. The hilt should fit easily in your hand, and the blade should not be so heavy that it tends to drag the hilt from your fingers in a loose grip. It is essential that the blade have a sharp stabbing point and good cutting edges, because an artery torn through (as against a clean cut) tends to contract and stop the bleeding. If a main artery is cleanly severed, the wounded man will quickly lose consciousness and die."

Fairfield himself had a colorful history.He served as an instructor in the Shanghai Municipal Police for many years and trained UK, US and Canadian Commando forces, along with Ranger candidates in close-combat, pistol-shooting, and knife-fighting techniques during WWII.

I have to include this excellent quote from Richard Dunlop's  Behind Japanese Lines :

British Major Fairbairn, who had been chief of police in Shanghai before the Japanese capture of the city, taught the Fairbairn method of assault and murder. His course was not restricted to Camp X, but later given at OSS camps in the United States. All of us who were taught by Major Fairbairn soon realized that he had an honest dislike for anything that smacked of decency in fighting.