He was a U.S. Army officer who had been developing both armed and unarmed combat courses for the army when he was recruited into the OSS in 1941 specifically to train spies and commandos in hand-to-hand fighting at the OSS "School for Spies and Assassins." (The location of which is now Camp David.) He was in on the coordination of all clandestine missions, debriefed the agents afterwards, and didn't hesitate to admit that he learned from them himself. (Like from the Finnish soldier who had killed 21 Russians with a knife...)
During the war, he wrote Kill or Get Killed, a classic how-to manual on close quarters combat. (An updated edition was published by the U.S. Marine Corps in 1976.) In 1969, he wrote Riot Control, Material and Techniques, and in 1993, Combat Use of the Double-Edged Fighting Knife. Not a guy to screw around with.
Among other things, Fairbairne developed the so-called "Sykes-Fairbairne Fighting Knife," made famous by the British Commandos during WWII, and shown at right. It had some inherent weaknesses, (like a tip that broke off too easily and a grip that slipped around in your hand) and Applegate made some improvements and called the result the "Applegate-Fairbairne Knife." (Be that as it may, the original Sykes-Fairbairne is still in production, and still in use with British SAS and SBS units, among others.)
Rex Applegate died at home in Yoncalla, Oregon in 1998. He was 84 years old.
Applegate trivia: At one point during WWII, he was the personal bodyguard of President Franklin D. Roosevelt. After the war, he served for 15 years as a military advisor to the government of Mexico, which made him an "Honorary General."
Later in life, he was a pal of John Wayne's and was a technical advisor on the set of The Alamo. (He was also the man who taught the Duke how to shoot.)