the bombing of Guernica - one of the most infamous incidents of the Spanish Civil War

The Condor Legion used Junkers Ju 52 and He-111 (shown) bombers

Prima facie in support of General Franco's insurrection in Spain during the Spanish Civil War, Hitler's Condor Legion of the Luftwaffe bombed the north Spanish town of Guernica on 26 April 1937. The bombardment destroyed numerous buildings, killing an estimated 1,650 people and injuring many more.  It was more than likely a trial run for the bombing of Polish cities in September 1939 when Hitler invaded that country.  Details here

It inspired Pablo Picasso to paint in Paris (where he lived from 1901) the large mural-sized 3.5 metres high and 7.8 metres wide artwork to commemorate it. From there the painting went on a tour in different countries before finishing up in New York. It stayed in the custody of the Museum of Modern Art until 1981 when it was delivered to Spain, reflecting Picasso's wishes that it not go there until Spain was a democracy. It is today in the Museo Reina Sofia in Madrid.