IN Room 38 of the Louvre’s Richelieu Wing hangs “The Astronomer” by the Dutch master Jan Vermeer. It is an exquisite painting. The stargazer sits before a celestial globe, his fingers spanning the constellation Pegasus. He wears a teal Japanese silk robe, a style favored by Dutch burghers in the late 17th century. He is lost in thought and bathed in a golden light.

Of course we can’t see the back of the painting. But if we could take it down from the wall and turn it over, we would find the spot where a small black swastika was stamped by Nazi curators after it was stolen from Édouard de Rothschild, a Jewish collector whose art had been coveted by Hitler since before the start of the war.

“The Astronomer” is but one of thousands of pieces of artwork in Paris that carry such a history. France was the most looted country during World War II, with over one-third of all privately owned art stolen. Seventy years after the fall of Paris, it is still possible to follow a trail of the city’s looted treasures, many of which have been recovered and returned to museums and collections all around the city.

In 2001 I visited Paris as a graduate student, with a plan to research this story of Nazi art theft for a novel. I can still recall a day when I stood in the Louvre’s Grand Gallery amid a mob of tourists. Only that morning I had been reading the autobiography of the Louvre curator and Resistance hero Rose Valland. In it, old black and white photographs showed the museum in late summer of 1939. The Louvre was shuttered, the great hall emptied of its Leonardos and Mantegnas and giant Veronese.