As a result of an arbitration issued by King Frederick Augustus I of Saxony in 1826 , the Ernestine duchies were rearranged and Saxe-Gotha-Altenburg was again split : After the abolition of German monarchies in the course of the German Revolution of 1918-1919 , all former duchies became part of the newly created state of Thuringia in 1920 .
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 .