It was one of the most horrific events in modern warfare. During the Spanish Civil War, at the behest of Franco, the German Luftwaffe bombed the Basque town of Gernika, on a Monday, the traditional market day for the town. They also bombed Durango and, I have read, the town where my dad is from, Gerrikaitz (though I have not found any details about that). Gernika was one of the first examples of aerial bombardment of a city to provoke terror in its citizens. Since then, there have been many other examples. See this Wikipedia article for more information about the bombing.
At about the same time, Picasso was asked to paint a large mural for Spain’s contribution to the World’s Fair to be held in Paris. Picasso ended up chosing the bombing of Gernika as his subject and painted his now famous and iconic Guernica. It is an impressive painting, currently hanging in the Reina Sofia museum in Madrid. It is huge, encompassing an entire wall in the museum. As with most works of art of this magnitude and importance, the meanings of the various elements — the bull, the horse, the various people — have been debated almost since Picasso first displayed the piece.
Author and Picasso expert Gijs van Hensbergen has recently written a new book about his interpretation of the meanings of those symbols, entitled Guernica: The Biography of a 20th Century Icon. In this BBC article, he describes what he believes the most significant elements of the painting mean.
Thanks to Jose Antonio Alcayaga for pointing me to this article.