Gladio (Italian, from Latin gladius, meaning sword) is a code name denoting the clandestine NATO "stay-behind" operation in Italy after World War II, intended to counter an eventual Warsaw Pact invasion of Western Europe. Although Gladio specifically refers to the Italian branch of the NATO stay-behind organisations, Operation Gladio is used as an informal name for all stay-behind organisations. Operating in all of NATO and even in some neutral countries or in Spain before its 1982 adhesion to NATO, Gladio was first coordinated by the Clandestine Committee of the Western Union (CCWU), founded in 1948. After the creation of NATO in 1949, the CCWU was integrated into the Clandestine Planning Committee (CPC), founded in 1951 and overseen by the SHAPE (Supreme Headquarters Allied Powers Europe), transferred to Belgium after France’s official withdrawal from NATO in 1966 — which was not followed by the dissolution of the French stay-behind paramilitary movements. According to historian Daniele Ganser, one of the major researchers on the field, "Next to the CPC, a second secret army command center, labeled the Allied Clandestine Committee (ACC), was set up in 1957 on the orders of NATO's Supreme Allied Commander in Europe (SACEUR). This military structure provided for significant US leverage over the secret stay-behind networks in Western Europe as the SACEUR, throughout NATO's history, has traditionally been a US General who reports to the Pentagon in Washington and is based in NATO's Supreme Headquarters Allied Powers Europe (SHAPE) in Mons, Belgium. The ACC's duties included elaborating on the directives of the network, developing its clandestine capability, and organizing bases in Britain and the United States. In wartime, it was to plan stay-behind operations in conjunction with SHAPE. According to former CIA director William Colby, it was 'a major program'." Ganser27.pdf" target="_blank">"Terrorism in Western Europe: An Approach to NATO’s Secret Stay-Behind Armies" Acrobat file _ETH Zurich research project on Gladio directed by Dr. Daniele Ganser

The role of the CIA in sponsoring Gladio and the extent of its activities during the Cold War era, and its relationship to attacks perpetrated in Italy during the years of lead and other similar clandestine operations is the subject of ongoing debate and investigation. Italy, Switzerland and Belgium have had parliamentary inquiries into the matter. The US State Department has denied involvement in terrorism and stated that some of the allegations and researchers, such as Ganser, have been influenced by what it and others describe as a Soviet forgery, US Army Field Manual 30-31B.