In 1914, the last Prussian king and German emperor, Wilhelm II Hohenzollern, signed a declaration of war against the Entente forces in the New Palace.

An excerpt from the article 24 facts about Potsdam

After the end of World War I, the era of monarchy in Germany ended, Wilhelm II abdicated and fled to the Netherlands in 1918. Potsdam eventually lost its status as a provincial capital, and Germany became a republic.