This week we explore some of the most powerful artworks ever made, making the case for political art one work at a time. Pablo Picasso's Guernica, Kathe Kollwitz's prints, Kazimir Malevich's Black Square, Iri and Toshi Maruki's Hiroshima Panels, and Martha Rosler's House Beautiful: Bringing the War Home photomontages. What do you think of as political art?