During the second world war these lads and their system trained thousand of clandestine operatives from the USA, Canada, Great Britian and France in hand to hand and edged weapon combat. They also invented a fantastic little knife appropriately named the Fairbairn-Sykes Fighting Knife.
A double edged dagger the philosophy behind the F-S Fighting Knife was to surprise the opponent. The Slender blade profile was meant to penetrate the ribcage and pierce throug bone wth ease. The philosophy is furthur explained by this excerpt from Fairbairns own book.
"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."
Included in the training system was the study of anatomy and target areas. A spreadsheet was drafted up showing the effectiveness of key striking points and the speed each point rendered death.
According to some old training pictures I found the primary deployments include the slashing at the arm in the vicinity of the elbow or rakeing down the arm toward the wrist in response to an opponent grabbing you, thrusting into the neck aiming for the common carotid artery and thrusting straight down into the area near the shoulder.
the F/S Fighting Knife remained in service for many years and today an original dagger makes for a fine edition to a collection. Modern working replicas are available though mileage will vary based on manufacturer.
A similar double edged dagger saw service with the unit that would become the precursor to all modern special operations forces in North America the joint Canadian-American unit the First Special Service Force, The blade developed by FSSF Officers became known as the V-42 Stiletto.
Developed primarily by Col Robert T Frederick of the FSSF who seeked a fighting knife that could silently eliminate enemy soldiers as well as perform the role of close-quarters combat. The V-42 was based off of features from the F-S fighting knife. Key differences from the F-S included a thinner stiletto blade which allowed optimized penetration. It is documented that the V-42 could easily penetrate an Issued US helmet with ease. An addition to the blade was a pommell on the hilt of the blade capable of caving in a foes skull. The thumb groove on the V-42 was designed to promote a flat grip with the thumb over the crossguard, which positioned the double-edged blade horizontally allowing smooth entry between ribs.
The sheath was abnormally long in design to incorporate the use of a cold weather parka due to the forces original training being based around winter operations.
Today the knife can be found on the crests of JTF-2 and the US Army Special Forces. Canada and the USA's premiere special operations forces.
alongside the use of fighting knives the second world war saw several unique E&E and last ditch knives one of which was the OSS thumb dagger seen in this photo alongsde a OSS sleeve dagger.
The concept behind the thumb dagger was to give an OSS Spy a last ditch knife to escape captivity with. The blade was typically sewn into a part of the agents clothing that would not be felt throughly during a frisk search. The concept of the thumb daggers deployment as a weapon was about about quickly enducing "psychological shock" into the foe quickly to eliminate the odds of the opponent chasing you. One of the key moves to do that was stabbing the foe in the eyeball and slash at the throat.
Primarly though the knife was used as a tool to escape captivity, cut through rope it was not exactly intended as a fighting knife. The OSS/SOE developed a plethora of unnique E&E knives.
Two unoffical copies of the thumb dagger exist today the Mantis MU-3 and the 888 Professional S.O.L
All in all I could write on WW2 blades forever and even just focusing on OSS/SOE knifes would eat up a lot of time but this is just to serve as a taste of the edged weapons of the Second World War.