On April 26, 1937, in the late afternoon of a busy market day in the Basque town of Gernika in northern Spain, the German Luftwaffe began the relentless bombing and machine-gunning of buildings and villagers. Three-and-a-half hours later, the village lay in ruins, its population decimated. This act of terror and unspeakable cruelty outraged the world and one man in particular, Pablo Picasso. The renowned artist, an expatriate living in Paris, reacted...