The palace on the mountain loomed over a stretch of rolling Bavarian countryside so lovely it was known as Gottesgarten—God’s Garden.

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.