The trials prosecuted 16 members of the Basque separatist organisation Euskadi Ta Askatasuna (ETA) for their involvement in two murders of police officers in 1968.

A labour strike by around 100,000 Basque workers, and the kidnapping by ETA of a German honorary consul, contributed to the significant media attention around the trials.

Sparking condemnations from public figures including Pope Paul VI and Jean-Paul Sartre, the Burgos trials gained notoriety for being "one of the last occasions on which political prisoners were sentenced to [death].

[2] Two months later, on 2 August 1968, ETA committed its first premeditated murder by assassinating Melitón Manzanas,[3] local commander of the Brigada Político-Social accused of torturing Basque detainees, at his residence in San Sebastián.

Intended as a retaliation for the death of Etxebarrieta, the murder provoked a harsh reaction from the Spanish authorities: constitutional rights for the province of Gipuzkoa were suspended and by the end of the year many members of ETA had been arrested.

[2] In August 1970, the Spanish government resolved to hold a military tribunal, whose task was to convict the members of the ETA involved in the crimes of the preceding years.

[8] Although mainstream media attempted to portray the defendants as "members of an isolated terrorist band",[9] the trial generated widespread popular support for ETA among the Basque public.

[5] Up until its beginning, ETA tried to compromise the trial through paramilitary action; according to Casanova, a tunnel that they had dug underneath the prison in which the defendants were held failed to break through a concrete wall.

In a 2015 book, the historian John Sullivan said: [The statements] seemed to show that ETA had been transformed into a Marxist-Leninist organization which, while it confined its activities to Euskadi, sympathised equally with the oppressed elsewhere in Spain, and had completely abandoned anti-Spanish chauvinism.

[17] According to historian Luis Castells, the trials thus became "a milestone in the anti-Franco struggle", generating "an unreleased mobilization in the Basque Country, in Spain and internationally".

When proceedings resumed the following day, the tribunal adopted a more rigid approach, suppressing statements not related to the accessions to prevent further digressions by the defendants.

Onaindia was overpowered quickly; during the struggle, the rest of the prisoners stood up and sang the Basque soldiers' anthem, and the incident became subject to significant foreign media coverage.

"[1] In the preface to a book (Le procès de Burgos) published soon after the events,[23] the philosopher Jean-Paul Sartre praised the defendants for showcasing the predicament of the Basque people to the world.