| Conventional Long Name: | Duchy of Saxe-Altenburg (1602–1918) Herzogtum Sachsen-Altenburg Free State of Saxe-Altenburg (1918–1920) Freistaat Sachsen-Altenburg |
| Common Name: | Saxe-Altenburg |
| Status: | Vassal |
| Empire: | Holy Roman Empire |
| Government Type: | Duchy (1602–1918) Republic (1918–1920) |
| Status Text: | State of the Holy Roman Empire, State of the German Confederation, State of the North German Confederation, State of the German Empire, State of the Weimar Republic |
| Date Start: | 7 July |
| Year Start: | 1602 |
| Date End: | November |
| Year End: | 1918 |
| Life Span: | 1602–1672 1826–1920 |
| Event Start: | Saxe-Weimar partitioned |
| Event1: | Personal union with Saxe-Gotha* |
| Date Event1: | 1672–1825 |
| Event2: | Ernestine duchies rearranged, duchy restored |
| Date Event2: | 12 November 1826 |
| Event End: | German Revolution |
| Event Post: | Merger of Thuringia‡ |
| Date Post: | 1920 |
| P1: | Saxe-Weimar |
| Flag P1: | Großherzogin Sachsen-Weimar.svg |
| S1: | Thuringia |
| Flag S1: | Flag of Thuringia (state).svg |
| Flag Type: | Flag[1] |
| Image Map Caption: | Saxe-Altenburg within the German Empire The Ernestine duchies after 1825, with Saxe-Altenburg in orange |
| Capital: | Altenburg |
| Title Leader: | Duke |
| Year Leader1: | 1603–1613 |
| Year Leader2: | 1908–1918 |
| Footnotes: |
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| Today: | Germany |
Saxe-Altenburg (German: Sachsen-Altenburg) was one of the Saxon duchies held by the Ernestine branch of the House of Wettin in present-day Thuringia.[2] It was one of the smallest of the German states with an area of 1323 square kilometers and a population of 207,000 (1905) of whom about one fifth resided in the capital, Altenburg. The territory of the duchy consisted of two non-contiguous territories separated by land belonging to the Principality of Reuss-Gera. Its economy was based on agriculture, forestry, and small industry. The state had a constitutional monarchical form of government with a parliament composed of thirty members chosen by male taxpayers over 25 years of age.
Saxe-Altenburg had an area of 1,323 km2 (510 sq. mi.) and a population of 207,000 in 1905. Its capital was Altenburg.
The duchy consisted of two separate areas: the Ostkreis, containing the cities of Altenburg, Schmölln, Gößnitz, Lucka und Meuselwitz (including the exclave of Mumsdorf), Roschütz, Hilbersdorf, Neukirchen by Waldenburg and Rußdorf by Chemnitz; and the Westkreis, which contained the cities of Eisenberg, Kahla, Orlamünde und Roda (including the exclave of Ammelstädt). The Ostkreis roughly corresponds to the modern Altenburger Land district of Thuringia, plus the area around Ronneburg in Greiz. The Westkreis is now mostly in Saale-Holzland district, with small portions in neighbouring districts.The duchy contained the Pleiße and Saale rivers.
The duchy had its origins in the medieval Burgraviate of Altenburg in the Imperial Pleissnerland (Terra Plisensis), a possession of the Wettin Margraves of Meissen since 1243. Following the partition of Leipzig in 1485, Altenburg fell to Ernst, Elector of Saxony, the progenitor of the Ernestine Wettins. After the Capitulation of Wittenberg in 1547, the area around Altenburg went to the Albertine Electorate of Saxony, but it was transferred to the Ernestine in the in 1554, and then to the Duchy of Saxe-Weimar following the in 1572.
When Johann Wilhelm's son and successor Friedrich Wilhelm I died in 1602, the Duchy of Saxe-Weimar passed to his younger brother Johann II. In 1603 Frederick William's eldest son Johann Philipp received the newly created Duchy of Saxe-Altenburg as compensation. It was an Imperial State in its own right, with a vote in the Reichstag, for much of the 17th century until the extinction of its ruling line in 1672 when it was inherited by Ernest I the Pious, the Duke of Saxe-Gotha, who had married the heiress.
Saxe-Altenburg thereafter remained part of Saxe-Gotha-Altenburg until the extinction of that house in 1825, when Gotha and Altenburg were divided up, with Gotha going to the Duke of Saxe-Coburg-Saalfeld and Altenburg to the Duke of Saxe-Hildburghausen, who in exchange gave up Hildburghausen to the Duke of Saxe-Meiningen. This family ruled the duchy until the end of the monarchies in the course of the German Revolution of 1918–19. The succeeding Free State of Saxe-Altenburg was incorporated into the new state of Thuringia in 1920.
Line extinct, inherited by Saxe-Gotha, thereupon Saxe-Gotha-Altenburg