Picasso’s Guernica is on display in Spain, where the painting’s anti-war message stands in stark contrast to the terrorist attacks endured regularly by Spaniards. Guernicaitself has been the target of violence over the years, to the extent that it used to be displayed only under heavy glass. These days, it hangs unprotected, and Michael Kimmelman says that public trust is what makes art, and the museums that house it, so uniquely human.