Frederick Augustus I (full name: Frederick Augustus Joseph Maria Anthony John Nepomuk Aloysius Xavier; German: Friedrich August Josef Maria Anton Johann Nepomuk Alois Xavier; Polish: Fryderyk August Józef Maria Antoni Jan Nepomucen Alojzy Ksawery Wettyn; 23 December 1750 – 5 May 1827) was King of Saxony (1805–1827) from the House of Wettin. He was also Elector Frederick Augustus III (Friedrich August III.) of Saxony (1763–1806) and Duke Frederick Augustus I (Polish: Fryderyk August I) of Warsaw (1807–1813).

Succeeding his father in 1763 as the elector Frederick Augustus III, he brought order and efficiency to his country's finances and administration. In foreign policy, he was neutralist but drifted towards Prussia, whose side he took in the Bavarian succession dispute (1778–79), when it prevented Bavaria's cession to Austria. For his cooperation he received substantial financial compensation from Prussia. In 1785, Frederick Augustus joined the Prussian-sponsored Fürstenbund (League of Princes), but remained neutral during the Austro-Prussian dispute in 1790. Offered the Polish crown in 1791, he declined as he feared that his "risky" politics may cause further damage to the Polish state, which was already weak and eventually stopped existing in 1795. The next year Saxony reluctantly joined the coalition against Revolutionary France but was defeated by 1796. Again entering the struggle on Prussia's side in 1806, after the decisive defeat at Jena in the same year, Frederick Augustus made peace with Napoleon, which secured the title of king of Saxony for him. A year later, Napoleon secured the Grand Duchy of Warsaw for him. Frederick Augustus remained a loyal ally to France even after the disastrous Russian campaign (1812–13). Although he had started half-hearted negotiations with Austria, he broke them off after the French victory at Lützen (May 1813). In the Battle of Leipzig (October 1813), however, his troops went over to Prussia and he was taken as prisoner to Berlin. At the Congress of Vienna in 1815, Frederick Augustus lost three-fifths of his territory to Prussia. He spent the rest of his life attempting to rehabilitate his truncated state.

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