Son and heir of Prussian king Frederick William II, he received a military education and held active commands during the War of the First Coalition from 1792 to 1794. King of Prussia on his father's death in 1797, he rescinded some of the monarchy's more repressive legislation, ans was less inclined to reactionary intolerance than his predecessor, but earned a reputation for indecision and dependance on the formidable Queen Louise.
Her ardent faith in Prussia's international destiny led the king into a disastrous entanglement with the Third Coalition in 1805, and the folly of a unilateral declaration of war against a victorious France in 1806. He was forced into virtual exile in the East Prussian port of Königsberg after the rout of his armies at Jena-Auerstädt in autumn 1806, and his subsequent role in international affairs reflected his military weakness.
Allowed back to Berlin by Napoleon in late 1809, he was still a reluctant French ally in 1812, and wavered in the face of overwhelming popular and political opposition to the alliance, even after the Russian campaign. The prospect of Russian troops reaching Berlin eventually outweighed his fear of Napoleon, and he accompanied his armies through the campaigns of 1813-14, but he remained an uncertain figure, dominated by the strategic dictates of Tsar Alexander I and the unbridled aggression of his field commanders. Ruler of a much enlarged kingdom after 1815, he followed Alexander's lead in joining the Holy Alliance of conservative monarchs, seeking to reinforce royal autocracy and to secure economic hegemony over northern Germany.
Source: Dictionary of the Napoleonic Wars, ed. S. Pope, London: Collins, 1999