Close Quarters Combat System (also known as Defendu) is a modern martial art developed by William E. Fairbairn and Eric A. Sykes prior to World War II.

It is a hand-to-hand combat system based on practical experience mixed with Jujutsu and boxing that was developed to train the Shanghai Municipal Police, and was later taught in expanded form to Office of Strategic Services and Special Operations Executive members during World War II.

[citation needed] This confused early readers of the book, who assumed that the techniques within had been based mainly in the Eastern martial arts that Fairbairn had learned.

The original Defendu was oriented towards self-defense and restraint, while the Close Quarters Combat system concentrated on rapid disabling of an opponent, with potentially lethal force.

Through Col. Applegate and other instructors such as Col. Anthony Biddle, these highly effective skills were taught to U.S. troops including US Marines and Rangers, as well as OSS operatives and later to the FBI and CIA as the foundation of their basic training.

Fairbairn's 'special assignment' in Oshawa, Ontario, Canada, was to teach his system to Allied special forces at the most highly classified training operation of WWII, Camp X.

Defendu encourages its practitioner to end a confrontation as quickly as possible using "rude" means by rapidly attacking vital spot area (such as the groin, throat, side of the neck, shin, eyes, ears, etc.)