Picasso was moved to paint the huge mural Guernica shortly after German planes, acting on orders from Spain's
                authoritarian leader Francisco Franco, bombarded the Basque town of Guernica on April 26, 1937, during the Spanish civil
                war. Completed in less than two months, Guernica was hung in the Spanish Pavilion of the Paris International Exposition
                of 1937. The painting does not portray the event; rather, Picasso expressed his outrage by employing such imagery as the
                bull, the dying horse, a fallen warrior, a mother and dead child, a woman trapped in a burning building, another rushing
                into the scene, and a figure leaning from a window and holding out a lamp. Despite the complexity of its symbolism, and
                the impossibility of definitive interpretation, Guernica makes an overwhelming impact in its portrayal of the horrors of war.
                It was on extended loan at New York City's Museum of Modern Art from 1939 until 1981, when it was returned to Spain at
                Madrid's Prado Museum. In 1992 the work was moved to the city's new museum of 20th-century art, the Reina Sofia Art
                Center. Dora Maar, Picasso's next companion to be portrayed, took photographs of Guernica while the work was in
                progress.
 

(Information from the bio page on Picasso at  Carol Gerten's Fine Art - A Virtual Art Museum)
 

For more information on Picasso, see The Online Picasso Project