26 April 1937,
They bombed Gernika. The massacre was carried out by the Nazi German Condor Legion and the Legionary Aviation of fascist Italy. This attack on market day civilians killed nearly 2,000 people. The Guernica painting, carried out by Pablo Picasso from the bombardment, contributed to the international expansion of the event. Francoists also bombed more towns and cities in the Basque Country.
Source:
Iñaki Egaña, The Crimes of Franco in Euskal Herria, 1936-1940. (Txalaparta).