[1] When Belgium was occupied by the troops of the German Empire during World War I, Rieth worked in the occupation administration.

In New York City, Rieth held negotiations with Walter C. Teagle, chairman of the board of the Standard Oil Company.

[4][5] Following a tip from William Samuel Clouston Stanger, Rieth was arrested in his quite at the Waldorf Astoria and interned at Ellis Island and deported by the U.S. Bureau of Immigration in early June 1941.

As a result, the Germans set up a consulate with around 50 diplomats, which Rieth served as acting director after his deportation from the United States.

At the beginning of February 1944, the German consulate in Tangier was closed by Spanish occupation authorities under Luis Orgaz Yoldi.