Bombing of Guernica
The Bombing of Guernica was an aerial assault on the town of Guernica in the Basque region of Spain during the Spanish Civil War. It was carried out by the German Luftwaffe and the Italian air force in support of fellow fascist Francisco Franco and his Nationalist insurgency.
It is an early example of terror bombing, and resulting in civilian casualties totalling 1654 dead, according to the Basque government. Some later historians and Wikipedia have challenged this figure, claiming the deaths to be no more than 200-400. For years the Spanish government claimed that the Basques themselves had dynamited the city.
Wikipedia's version of the death count relies mainly on some thesis written by a guy in Alabama who thinks that the body count per ton of bombs cannot be four times what it was during bombings later in the war. He ignores the facts that civilians at the time might not have been trained to take cover from aerial bombardment and that many buildings in Spain might lack basements, such as existed in England and Germany and doubtlessly saved many lives.
Pablo Picasso made a famous painting about this incident and the horror of war.
The bombing techniques developed during the Spanish Civil War were later used by the Germans and Italians during World War II, and the psychological threat of terror bombing weakened Allied resolve in France, Belgium, and the Netherlands.