On 15 June (O.S.) / 25 June (N.S.) 1656, Charles X Gustav and Frederick William I concluded a formal alliance in the Treaty of Marienburg after Swedish advances in the Polish–Lithuanian Commonwealth had stalled. Sweden offered conquered Polish territories, and Frederick William I went to support Charles X Gustav with his newly formed army in the Battle of Warsaw on 28–30 July, which marked "the beginning of Prussian military history".