The Basques are an ethnic group with their own separate language and culture indigenous to the western Pyrenees, the mountain range that forms much of the border between Spain and France. Basque nationalists have sought an independent Basque state since the late 19th century.
ETA was formed in Spanish Basque territory 1959, and over the next four decades, it pursued a campaign of violence and assassination that fueled the development of far-right opposition groups that added to the bloodshed.
In 1973, ETA militants set off a bomb near a church where Spanish Prime Minister Luis Carrero Blanco was attending Mass. Blanco — who was widely seen as the heir-apparent to Spain's longtime right-wing dictator, Francisco Franco — was killed.
Spain made the transition to democracy after Franco died in 1975, but ETA pushed on with its campaign.
Its assaults included other assassinations, as well as scores of mass bombings, among them explosions that killed 12 civil guards in Madrid in June 1986, 21 people in Barcelona in June 1987 and 11 people at a barracks in Zaragoza in December 1987.
In 1995, ETA again targeted the top of the government, detonating a bomb in April near the car carrying Prime Minister Jose Maria Anzar, who was saved by its protective armor, and hatching a plot to assassinate King Juan Carlos, which police said they broke up in August.
Thursday's van attack occurred in Catalonia, a large separate autonomous region on the northeast tip of Spain that was oppressed during Franco's 36 years in power.
Catalonia, Spain's most prosperous region, is also the center of a vigorous independence movement, but it has historically used political and cultural tools, not violence.
As well as at the ballot and in the courtroom, the Catalan independence movement is vividly expressed in the rivalry between two of the biggest soccer clubs in the world: FC Barcelona, which sports Catalan symbols on its uniforms and promotes itself specifically as a Catalan club, and Real Madrid CF, in the capital.
Concrete ties between Real Madrid and the Franco regime have almost certainly been exaggerated — many European soccer historians argue that they could be described only as semi-official. But it was Franco himself who restored the Real (which means "royal" in English) to Madrid FC's name when he assumed power in 1939.
Alex Johnson is a reporter and editor for NBC News based in Los Angeles.