Original: After the abolition of German monarchies in the course of the German Revolution of 1918-19, all former duchies became part of the newly created state of Thuringia in 1920.
Simplified: After the abolition of German monarchies at the end of the First World War, both Saxe-Gotha and Saxe-Altenburg became part of the newly created state of Thuringia in 1920.