Yves Guérin-Sérac, real name Yves Guillou (alias Jean-Robert de Guernadec, alias Ralf) (born 1926 in Ploubezre) was a French anti-Communist Roman Catholic activist, former officer of the French army and veteran of the First Indochina War (1945–54), the Korean War (1950–53) and the Algerian War of Independence (1955–62). He was also a member of the elite troop of the 11ème Demi-Brigade Parachutiste du Choc, which worked with the SDECE (French intelligence agency), and a founding member of the Organisation armée secrète (OAS), a French terrorist group based in Spain which fought against Algerian independence in 1961-62. It was alleged that he was an instigator of the so-called strategy of tension in Italy, and the main organizer of the 1969 Piazza Fontana bombing.
In June 1962, after the 18 March 1962 Évian Accords that put an end to the Algerian War, Guérin-Sérac was hired by Franco to engage in operations against the Spanish opposition. He then worked for Salazar's Estado Novo regime in Portugal, which, beside being the last colonial empire, was also, in his eyes, the last stronghold against communism and atheism: "The others have laid down their weapons, but not I. After the OAS I fled to Portugal to carry on the fight and expand it to its proper dimensions - which is to say, a planetary dimension."  Guérin-Sérac met Petainist Jacques Ploncard d'Assac in Portugal, who introduced him to the right-wing establishment and to Portugal's secret police, the PIDE. Due to his extensive knowledge, Guérin-Sérac was recruited as an instructor for the paramilitary Legião Portuguesa, and for the counterguerrilla unit of the Portuguese army.
According to the magistrate Guido Salvini, in charge of the investigations concerning the 1969 Piazza Fontana bombing, "Guido Giannettini had contacts with Yves Guérin-Sérac in Portugal ever since 1964." 
It was within this context that in 1965 he founded, along with Italian neofascist Stefano Delle Chiaie, Aginter Press, a secret anti-communist army, with the support of both the PIDE and the U.S. Central Intelligence Agency. A pseudo-press agency, "[It]... set up training camps, in which it instructed mercenaries and terrorists in a three-week course in covert action techniques, including hands-on bomb terrorism, silent assassination, subversion techniques, clandestine communication and infiltration, and colonial warfare."  "During this period, disclosed Guérin-Sérac, we have systematically established close contacts with like-minded groups emerging in [current and former colonial powers] Italy, Belgium, Germany, Spain and Portugal, for the purpose of forming the kernel of a truly Western League of Struggle against Marxism."  On 31 January 1968, Guérin-Sérac met Pino Rauti, then leader of Ordine Nuovo, who would rejoin the fascist Italian Social Movement (MSI) the following year. In the 1970s, Guérin-Sérac was in contact with Leo Negrelli, former chief press attaché of the failed German puppet state, the Italian Social Republic.
- Daniele Ganser (2005), Operation Gladio and Terrorism in Western Europe, London, Franck Cass, 2005, p.116
- "L'orchestre noir", Film Documentary (2x55') (1997). Investigations by Fabrizio Calvi and Frédéric Laurent. Realisation by Jean-Michel Meurice. Production La Sept Arte/LP Productions/Rai Due. See here (in French)
- Paris Match, November 1974, quoted in Stuart Christie, Stefano Delle Chiaie (London, Anarchy Publications, 1984, p.27)
- Judge Guido Salvini hearing before the Italian Parliamentary Commission of investigation on terrorism in Italy, 9th session of 12 February 1997 (9ª SEDUTA - MERCOLEDI 12 FEBBRAIO 1997, Presidenza del Presidente PELLEGRINO (in Italian), quoted by Daniele Ganser, NATO's Secret Armies: Operation Gladio and Terrorism in Western Europe, ISBN 0-7146-8500-3 (2005), p.120)
- D. Ganser, op.cit, p.117
- Stuart Christie, ibid.
- Mort (non-accidentelle) d'un anarchiste Archived September 30, 2007, at the Wayback Machine, Le Courrier, May 3, 2005 (in French)
- Corrado Incerti, Sandro Ottolenghi and Piero Raffaelli GIORNALISTI ITALIANI AL SERVIZIO DELL'AGENZIA TERRORISTICA, L'europeo, 1974 (in Italian)