In early 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. 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 Kolin in Bohemia. Austrian Marshal 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 Daun away, and Frederick was forced to attack prepared positions. The resulting Battle of Kolin was a sharp defeat for Frederick, his first military defeat. His losses further forced him to lift the siege and withdraw from Bohemia altogether.
Why did Frederick reduce his manpower 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 Kolin in Bohemia