Original: 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.
Simplified: 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.