From the villages and farmsteads on the meandering river below, Schloss Banz commanded attention. Its sprawling stone wings glowed a luminous gold in the sunlight, and a pair of delicately tapered copper spires rose high above its Baroque church. The site had a thousand-year history: as a trading post, as a castle fortified to withstand armies, as a Benedictine monastery. It had been pillaged and destroyed in war, and extravagantly rebuilt for the royal Wittelsbach family. Kings and dukes, and once even Kaiser Wilhelm II, the last emperor of Germany, had graced its opulent halls. Now, in the spring of 1945, the colossus was an outpost of a notorious task force that had spent the war looting occupied Europe for the glory of the Third Reich.

As defeat drew near following six punishing years of war, Nazis all across Germany had been burning sensitive government files before the documents could be seized and used against them. But bureaucrats who could not bring themselves to destroy their papers instead hid them in forests, in mines, in castles, and in palaces like this one. Around the country, immense libraries of secrets were there for the Allies to find: detailed internal records shedding light on the warped German bureaucracy, on the military’s pitiless war strategy, and on the obsessive Nazi plan to clear Europe of its “undesirable elements,” finally and forever.

In the second week of April, the soldiers of General George S. Patton’s Third U.S. Army and General Alexander Patch’s Seventh U.S. Army overran the region. Since crossing the Rhine a few weeks earlier, the men had charged across the western reaches of the battered country, slowed only by demolished bridges, improvised roadblocks, and pockets of stubborn resistance. They passed cities flattened by Allied bombs. They passed hollow-eyed villagers and houses flying not the Nazi swastika but white sheets and pillowcases. The German army had all but disintegrated. Hitler would be dead in three and a half weeks.

Not long after the Americans arrived in the region, they encountered a flamboyant aristocrat who wore a monocle and high, polished boots. Kurt von Behr had spent the war in Paris plundering private art collections and ransacking common household furnishings from tens of thousands of Jewish properties in France, Belgium, and the Netherlands. Just before the liberation of Paris, he and his wife fled to Banz with loads of pilfered treasure in a convoy of eleven cars and four moving vans.

Now von Behr wanted to cut a deal.

He went to the nearby town of Lichtenfels and approached a military government officer named Samuel Haber. It seemed that von Behr had grown accustomed to living like royalty beneath the elaborately painted ceilings of the palace. If Haber would give him permission to stay put, von Behr would show him a secret stash of important Nazi papers. ...