He showed up at the Museum again in October 1939 with the Gestapo, after the capitulation of the Polish capital, and directed the SS-Untersturmführer Theo Daeisel to art pieces that needed to be seized and shipped to Germany first, including the Portrait of a Young Man by Rembrandt from the collections of the Łazienki Palace (Pałac Łazienkowski),[2] and numerous other masterpieces including paintings by Bernardo Bellotto called Canaletto.
[1] The items stolen from the National Museum included 99% of all coins, 100% of historic clocks, 80% jewellery, 63% fabrics, 60% furniture, and 70% ancient manuscripts.
Together, they pinpointed all sorts of treasures at Wawel (architectural detail notwithstanding), directing Frank in a mass looting campaign.
In 1929 he published his controversial book about the Late Middle Ages and in 1931 moved to the Weimar Republic to teach art history in Breslau (Wrocław).
In 1941 he wrote his infamous book Krakau (German for Kraków) – denying this ancient Polish capital its Slavic origins.