Nevertheless, Albert died in 1644, and Ernest inherited large parts of his duchy, though not the core territory around the residence at Eisenach and the Wartburg, which fell to his elder brother Wilhelm of Saxe-Weimar. Ernest could also incorporate several remaining estates of the extinct House of Henneberg in 1660, which had been vacant since 1583. Finally in 1672 he received the major part of Saxe-Altenburg through his wife Elisabeth Sophie, after Altenburg's last duke Frederick William III had died without heirs. Ernest would then be called Duke of Saxe-Gotha-Altenburg.
When Ernest died in 1675, he left his seven sons a significantly enlarged territory. The eldest, Frederick I at first ruled jointly with his brothers until in 1680 the duchy was divided. The area around Gotha and also Altenburg passed to Frederick I, who retained the title of a Duke of Saxe-Gotha-Altenburg. For later history of the duchy, see Saxe-Gotha-Altenburg.
When the house of Saxe-Gotha and Altenburg became extinct in 1825, Saxe-Gotha-Altenburg was split. Saxe-Gotha passed to the Duke of Saxe-Coburg-Saalfeld who in turn gave Saalfeld to Saxe-Meiningen. The Duke of Saxe-Hildburghausen received Saxe-Altenburg, and gave the district of Hildburghausen to Saxe-Meiningen.
After the abolition of German monarchies at the end of the First World War it became a part of the newly created state of Thuringia in 1920.
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