Wilhelm I 's grandson Kaiser Wilhelm II abdicated in 1918 after Germany 's defeat in the First World War . King Frederick Augustus III of Saxony followed him into abdication and the Kingdom of Saxony became the Free State of Saxony inside the newly-formed Weimar Republic . It became a Free state in the era of Weimar Republic in 1918 after the end of World War I and the abdication of King Frederick Augustus III of Saxony .