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Saxe-Gotha-Altenburg (German: Sachsen-Gotha-Altenburg) was a country in what is today Thuringia, Germany.
It was formed in 1672 when Frederick Wilhelm III, the last duke of Saxe-Altenburg died and Ernest I, Duke of Saxe-Gotha (who had married Frederick Wilhelm's cousin, Elisabeth Sophie) inherited his possessions.
The duchy was split up again after Ernest’s death in 1675, and the Duchy of Saxe-Gotha-Altenburg officially came into existence in 1680 when his eldest son, Frederick, took control of the parts of the country Gotha and Altenburg.
When the royal house of Saxe-Gotha and Altenburg became extinct in 1825, Saxe-Gotha was given to Saxe-Coburg-Saalfeld and Saxe-Altenburg passed to the Duke of Saxe-Hildburghausen. 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.
Dukes of Saxe-Gotha-Altenburg
Divided between Saxe-Coburg-Saalfeld and Saxe-Hildburghausen
Duchies: Anhalt: Bernburg, Dessau, Köthen | Arenberg | Mecklenburg: Schwerin, Strelitz | Nassau | Oldenburg | Saxony: Coburg-Saalfeld, Gotha-Altenburg, Hildburghausen, Meiningen, Weimar*, Eisenach*, Weimar-Eisenach**
Principalities: Hohenzollern: Hechingen
| Reuß: Ebersdorf
| Salm: Kyrburg
| Schwarzburg: Rudolstadt