The Museum Boijmans flourished under his directorship, but he was also arrested and interned for eight months for his conduct during the German occupation of the Netherlands during World War II.
However, a quarter century after his death, he was at least partially vindicated when Le Blute-Fin Mill, a painting he had championed (to universal scorn) as a van Gogh, was finally authenticated as being by the renowned painter.
He was also instrumental in the construction of a new building, built by architect A. van der Steur in close collaboration with Hannema.
[2] The following year, he became a member of the Kultuurraad or "Culture Council", an advisory group of 23 pro-German "wise men" in the arts and sciences, and was put in charge of the country's museums in 1943 by Reichskommissar Arthur Seyss-Inquart.
[3] After the liberation of the country by the Allies, Hannema was arrested and interned for eight months for the illegal sale of art to the Germans, but was never formally convicted and was released in 1947.
He founded the Museum de Fundatie in Zwolle and bequeathed the painting to it[5] when he died at Wijhe, Netherlands in 1984.