The Spanish Civil War, fought from 17 July 1936 to 1 April 1939, was used by Germany and Russia to test new concepts in warfare including the use of bombers against cities.
In November 1936 German and Nationalist Spanish bombers attacked Madrid, held by Spanish Republican forces. The Spanish town of Guernica was subject to severe bombing on 26 April 1937. The cities of Barcelona and Valencia were also later attacked. Barcelona received a particularly severe attack lasting three days beginning with 16th March 1938, during which the famous Barcelona Cathedral was damaged and over a thousand civilians were killed or injured.
The Spanish Civil War set the scene for terror bombing that was to be repeated over and over during the Second World War on a scale that was inconceivable during the 1930's.