Wikipedia
Prinzenerlass ("the Princes' decree") was the name of a decree issued in 1940 by Adolf Hitler that prohibited all members of Germany's formerly reigning royal houses from joining or participating in any military operations in the Wehrmacht.
Until the Nazi seizure of power, and even until the outbreak of World War II, the members of the former German nobility continued to enjoy the rights they had prior to the abolition of the monarchy in 1918.
In May 1940, Prince Wilhelm of Prussia, the grandson of Kaiser Wilhelm II, took part in the invasion of France. He was wounded during the fighting in Valenciennes and died in a field hospital in Nivelles on 26 May 1940. His funeral service was held at the Church of Peace, and he was buried in the Hohenzollern family mausoleum in the Antique Temple in Sanssouci Park. The service drew over 50,000 mourners.
His death and the ensuing sympathy of the German public toward a member of the former German royal house greatly bothered Hitler, and he began to see the Hohenzollerns as a threat to his power. Shortly afterwards, the Prinzenerlass was issued, and all members of the former German royal houses were relieved from combat duties.