Years later, when ETA turned into a terrorist movement targeting democrats, Onaindia needed armed bodyguards in order to pursue his political goals. He lost many friends to assassins from an organisation for which he had once been prepared to die.
Born in Lekeitio, Vizcaya, Onaindia became a bank clerk after leaving school. Initially a member of the Basque Nationalist party, he became active on the left and, from 1967, was a leading figure in ETA, then a fringe group that emerged from student circles dissatisfied with the perceived quietism of traditional nationalism. Onaindia sought to develop a fusion of Marxist and nationalist discourse.
In 1968, ETA carried out its first assassination, of the notorious secret police torturer Melitón Manzanas, in San Sebastián. Following a roundup of suspects, Onaindia was one of 16 defendants in the infamous Burgos trial of 1969-70. Though there was no evidence of his direct involvement, he was among the six to receive death sentences.
Onaindia never expressed regret for having joined ETA, but maintained that the organisation bearing that name today is utterly different from the one in which he had been involved. As a bitter adversary of contemporary ETA, he maintained: "I fought fascism then, and I'm still fighting fascism today."
In the face of international protests, the Burgos death sentences were eventually commuted to exile. Onaindia himself served eight years in Spanish prisons, before settling in Belgium until it was safe to return home. The transition to a democratic monarchy in Spain, with recognition of the right of historic nationalities to a high level of self-government, persuaded him - and many other Basque leftists - to renounce violence and engage in civil politics. He became a lucid and passionate critic of ethnic nationalism in all its forms.
In 1977, he was the founding leader of a new leftwing party, EIA. This evolved into EE (Basque Left), and he championed an eventual merger with the Basque region of the mainstream socialists to form the PSE-EE.
In the early 1980s, along with the lawyer Juan Bandrés, Onaindia played a crucial role in negotiations that led ETA's so-called political-military faction to abandon arms. Elected to the Basque regional parliament in 1980, 1984 and 1986, he represented Guipúzcoa in the Spanish senate for two terms from 1993.
He held doctorates in English and Spanish, and was a prolific writer in the fields of politics, cinema, fiction and autobiography. One of his television projects was a compelling documentary series on the rise and decline of west European terrorism. He wrote a bestselling book offering perplexed Spanish readers A Guide To The Basque Labyrinth, and his memoirs, The Price Of Freedom (2001), gave a thoughtful insight into ETA's early years.
Today's ETA, Onaindia insisted, deserved to be classed as having been racist and fascist since around 1992. It was plain by then that most Basques approved of the autonomous regime within the Spanish constitution, so ETA's prime targets became Basque socialists, conservatives and journalists, rather than the Spanish state.
Onaindia suffered a heart attack in 1998 while playing the characteristic Basque sport of pelota, and had to stand back from the political frontline thereafter, taking up the provincial presidency of his party late in 2000. Cancer was diagnosed the following year.
He leaves his wife Esozi, daughter Nora and son Jon.
- Mario Onaindia Natxiondo, politician and writer, born 13 January 1948; died 31 August 2003