After the Soviet invasion of Poland in 1939, he was arrested by NKVD. Imprisoned in the infamous Lubyanka prison, he was personally interrogated by Lavrentiy Beria. He was released after a few months after international pressure from, among others, the Italian royal family (due to the prestige of the Radziwiłł family). He returned to Nazi occupied Poland, where he tried to use his prestige to improve Nazi treatment of the Poles; he met with Hermann Göring (whom he knew from before the war) but his efforts were futile. He was briefly imprisoned by the Germans during the Warsaw Uprising in 1944.