Frederick William III (German: Friedrich Wilhelm III) (3 August 1770 – 7 June 1840) was king of Prussia from 1797 to 1840. He ruled Prussia during the difficult times of the Napoleonic Wars and the end of the Holy Roman Empire. Steering a careful course between France and her enemies, after a major military defeat in 1806, he eventually and reluctantly joined the coalition against Napoleon in the Befreiungskriege. Following Napoleon's defeat he was King of Prussia during the Congress of Vienna, which assembled to settle the political questions arising from the new, post-Napoleonic order in Europe. Frederick William was born in Potsdam in 1770 as the son of Frederick William II of Prussia and Frederika Louisa of Hesse-Darmstadt. He was considered to be a shy and reserved boy, which became noticeable in his particularly reticent conversations distinguished by the lack of personal pronouns. This manner of speech subsequently came to be considered entirely appropriate for military officers. As a soldier he received the usual training of a Prussian prince, obtained his lieutenancy in 1784, became a lieutenant colonel in 1786, a colonel in 1790, and took part in the campaigns against France of 1792–1794. On 24 December 1793, Frederick William married Louise of Mecklenburg-Strelitz, who bore him ten children. In the Kronprinzenpalais (Crown Prince's Palace) in Berlin, Frederick William lived a civil life with a problem-free marriage, which did not change even when he became King of Prussia in 1797. His wife Louise was particularly loved by the Prussian people, which boosted the popularity of the whole House of Hohenzollern, including the King himself. Frederick William III succeeded to the throne on 16 November 1797. He also became, in personal union, the sovereign prince of the Principality of Neuchâtel (1797–1806 and again 1813–1840). At once, the new King showed that he was earnest of his good intentions by cutting down the expenses of the royal establishment, dismissing his father's ministers, and reforming the most oppressive abuses of the late reign. At first Frederick William and his advisors attempted to pursue a policy of neutrality in the Napoleonic Wars. Although they succeeded in keeping out of the Third Coalition in 1805, eventually Frederick William was swayed by the attitude of the queen, who led Prussia's pro-war party, and entered into war in October 1806. On 14 October 1806, at the Battle of Jena-Auerstädt, the French effectively decimated the effectiveness and functionality of the Prussian army led by Frederick William, and the Prussian army collapsed entirely soon after. The royal family fled to Memel, East Prussia, where they fell on the mercy of Emperor Alexander I of Russia.

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