Why is this painting so shocking?

In 1937, in one of the worst civilian casualties of the Spanish Civil War, Fascist forces bombed the village of Guernica in Northern Spain. For Pablo Picasso, the tragedy sparked a frenzied period of work in which he produced a massive anti-war mural, titled "Guernica." How can we make sense of this overwhelming image, and what makes it a masterpiece of anti-war art? Iseult Gillespie investigates. [Directed by Avi Ofer, narrated by Adrian Dannatt, music by Marcos Tawil].