On 18 April 1757, Frederick II again took the initiative by marching into the Kingdom of Bohemia, hoping to inflict a decisive defeat on Austrian forces. After winning the bloody Battle of Prague on 6 May 1757, in which both forces suffered major casualties, the Prussians forced the Austrians back into the fortifications of Prague. The Prussian army then laid siege to the city. In response, Austrian commander Leopold von Daun collected a force of 30,000 men to come to the relief of Prague. Following the battle at Prague, Frederick took 5,000 troops from the siege at Prague and sent them to reinforce the 19,000-man army under the Duke of Brunswick-Bevern at Kolín in Bohemia. Von Daun arrived too late to participate in the battle of Prague, but picked up 16,000 men who had escaped from the battle. With this army he slowly moved to relieve Prague. The Prussian army was too weak to simultaneously besiege Prague and keep von Daun away, and Frederick was forced to attack prepared positions. The resulting Battle of Kolín was a sharp defeat for Frederick, his first. His losses further forced him to lift the siege and withdraw from Bohemia altogether.