Complex sentence: After Germany had lost World War I (1914--1918), the German Revolution of 1918--1919 ended the monarchy and the German Empire was abolished and a democratic system, the Weimar Republic, was established in 1919 by the Weimar National Assembly.
Split into simpler sentences: After Germany had lost World War I (1914--1918), the German Revolution of 1918--1919 ended the monarchy and the German Empire was abolished. A democratic system, the Weimar Republic, was established in 1919 by the Weimar National Assembly.