Captain Robert Posey and Pfc. Lincoln Kirstein were the first through the small gap in the rubble blocking the ancient salt mine at Altausee, high in the Austrian Alps in 1945 as World War II drew to a close in May 1945. They walked past one sidechamber in the cool damp air and entered a second one, the flames of their lamps guiding the way. There, resting on empty cardboard boxes a foot off the ground, were eight panels of The Adoration of the Lamb by Jan van Eyck, considered one of the masterpieces of 15th-century European art. In one...