Applegate began hunting and shooting at a young age and learned marksmanship from his uncle Gus Peret who was a famed exhibition shooter and professional hunter at the time.

He was the close-combat coordinator for all clandestine missions and this role brought him into contact with other fighters and martial artists of the time period such as a Finnish soldier who killed 21 Russians with a knife, and the founder of the British SAS: David Stirling.

[1][2] After the close of World War II, Applegate spent the next 15 years as an advisor to the government of Mexico who made him an "Honorary General".

From the foreword: Fleet Marine Force Reference Publication (FMFRP) 12-80, Kill or Get Killed, is published to ensure the retention and dissemination of useful information which is not intended to become doctrine or to be published in Fleet Marine Force manuals.and This reference publication was written in 1976 by Lieutenant Colonel Rex Applegate, USA (Ret), with the help of the Combat Section, Military Intelligence Training Center, Camp Ritchie, Maryland.

At last there is one volume which speaks to the subjects of unarmed combat (offensive and defensive), combat use of weapons, disarming the enemy, handling of prisoners, the handle of mob/crowd disobedience, the use of chemicals in such situations, and how to establish a professional riot control unit.Applegate developed the techniques outlined in the book during his work with William E. Fairbairn, who had previously developed his own techniques while working for the Shanghai Municipal Police from 1907 to 1940.

Fairbairn drew heavily on Chinese martial arts, which he simplified and tailored to the needs of police training in one of the world's most crime-ridden cities, due to its history of crime related to the opium trade, the rebellion, and the activities of Triad gangsters.

Supporters of Cooper's methods point to the near-universal use of his system in the International Practical Shooting Confederation and other forms of action shooting, while Applegate's supporters point to police incidents where officers trained in Cooper's methods discharge many rounds at close range (most measured in tens of feet (3 m) or less) with few (if any) disabling hits.

Gerber Legendary Blades produced a line of folding knives by Applegate and Bill Harsey, Jr. based on the Applegate-Fairbairn fighting knife.