Special forces and special operations forces are military units trained to conduct special operations.

Special forces emerged in the early 20th century, with a significant growth in the field during the Second World War, when "every major army involved in the fighting" created formations devoted to special operations behind enemy lines

In mid-1942, Major-General Lucian Truscott of the U.S. Army, a liaison officer with the British General Staff submitted a proposal to General George Marshall that an American unit be set up "along the lines of the British Commandos", resulting in the formation of the United States Army Rangers.