Prussia joined Britain and Russia, thus forming the Fourth Coalition. Although the Coalition was joined by other allies, the French Empire was also not alone since it 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.