After the Anschluß of Austria to Nazi Germany in March 1938, he was arrested at the airport at Aspern and taken into custody by the Nazis because he was a Jew. While imprisoned he was visited by Heinrich Himmler. Rothschild apparently impressed the SS leader, who subsequently ordered that Rothschild's prison conditions be improved with better furniture and sanitation facilities. Despite appeals from Queen Mary of the United Kingdom and possibly the Duke of Windsor, Rothschild was held in Vienna's Hotel Metropole while the German government attempted to expropriate his business concerns. He was imprisoned at least through July 1938, and his property placed under control of a German "commissioner".Felix Somary, in his memoirs, recalls that, soon before the Anschluss, he phoned to the baron repeatedly, in a desperate attempt to convince him to leave Austria. The day before the Anschluss, Louis's brother Alphons and his wife were visiting him in Switzerland, wanting to go back into Austria; he persuaded them to remain there, and to get his children Francesca de Rothschild and Heidi de Rothschild away from Austria to Netherlands.
All of the Rothschild possessions were plundered and subsequently "Aryanised". The city-palace of the family was destroyed after the war. The baron never received most of his former belongings back, since most of the paintings were taken over by the Austrian state, which did not allow the paintings to leave the country.In 1998, over 200 art works were returned to the Rothschild heirs by the Austrian Government, and were placed at Christie's in London for auction in 1999.
In 1946 he married the countess Hildegard Johanna von Auersperg (1895–1981) and lived in East Barnard, Vermont (USA) and England.
^Regarding personal names: Freiherr was a title before 1919, but now is regarded as part of the surname. It is translated as Baron. Before the August 1919 abolition of nobility as a legal class, titles preceded the full name when given (Graf Helmuth James von Moltke). Since 1919, these titles, along with any nobiliary prefix (von, zu, etc.), can be used, but are regarded as a dependent part of the surname, and thus come after any given names (Helmuth James Graf von Moltke). Titles and all dependent parts of surnames are ignored in alphabetical sorting. The feminine forms are Freifrau and Freiin.
^MacDonogh, G. 1938: Hitler's Gamble. New York: Basic Books, 2009. p 61.