On 20 December 1973, Luis Carrero Blanco, the Prime Minister of Spain, was assassinated when a cache of explosives in a tunnel set up by the Basque separatist group ETA was detonated. The assassination, also known by its code name Operación Ogro, is considered to have been the biggest attack against the Francoist State since the end of the Spanish Civil War in 1939 and had far-reaching consequences within the politics of Spain.