By 1799 Napoleon had seized power in France and proved highly adept at warfare and coalition building. Britain led a series of shifting coalitions to oppose him. After a brief truce in 1802–3, war resumed. In 1806 Prussia joined Britain and Russia, thus forming the Fourth Coalition. Napoleon was not alone since he now had a complex network of allies and subject states. The largely outnumbered French army crushed the Prussian army at Jena-Auerstedt in 1806; Napoleon captured Berlin and went as far as Eastern Prussia. There the Russian Empire was defeated at the Battle of Friedland (14 June 1807). Peace was dictated in the Treaties of Tilsit, in which Russia had to join the Continental System, and Prussia handed half of its territories to France. The Duchy of Warsaw was formed over these territorial losses, and Polish troops entered the Grande Armée in significant numbers.