Bennigsen shifted his army north into East Prussia and launched a stroke at the French strategic left wing. The main force of the blow was evaded by the French at the Battle of Mohrungen in late January 1807. In response, Napoleon mounted a counterattack designed to cut off the Russians. Bennigsen managed to avoid entrapment and the two sides fought the Battle of Eylau on 7 and 8 February 1807. After this indecisive bloodbath both sides belatedly went into winter quarters. In early June, Bennigsen mounted an offensive that was quickly parried by the French. Napoleon launched a pursuit toward Königsberg but the Russians successfully fended it off at the Battle of Heilsberg. On 14 June, Bennigsen unwisely fought the Battle of Friedland with a river at his back and saw his army mauled with heavy losses. Following this defeat, Alexander was forced to sue for peace with Napoleon at Tilsit on 7 July 1807, with Russia becoming Napoleon's ally. Russia lost little territory under the treaty, and Alexander made use of his alliance with Napoleon for further expansion. Napoleon created the Duchy of Warsaw out of former Prussian territory.