Groups advocating the return of art looted by the Nazis in World War II have made great progress in recent years, but the battle is far from over. Just identifying looted art is a major undertaking: “One thorny issue is just how many looted items could have made their way to American museums. Under scrutiny are objects that were created before 1946 and obtained by a museum after 1932. Other criteria are whether the piece was in Europe at that time and whether ownership changed between 1932 and 1946.”