Museums in New York that exhibit artworks looted by Nazis during the Holocaust are now required to let the public know about that dark history through signs or placards put on display with the stolen objects. Experts estimate that at least 600,000 pieces of artwork were looted from Jewish people before and during World War II. Some of those objects ended up in the world's great museums. The new rule comes as many museums in the U.S. and Europe are reckoning with collections that also contain numerous objects looted from Asia, Africa and other places during centuries of colonialism.