Brandenburg-Prussia's new army survived its trial by fire through victory in the 1656 Battle of Warsaw, during the Northern Wars. Observers were impressed with the discipline of the Brandenburger troops, as well as their treatment of civilians, which was considered more humane than that of their allies, the Swedish Army. Hohenzollern success enabled Frederick William to assume sovereignty over the Duchy of Prussia in the 1657 Treaty of Wehlau, by which Brandenburg-Prussia allied itself with the Polish–Lithuanian Commonwealth. Despite having expelled Swedish forces from the territory, the elector did not acquire Vorpommern in the 1660 Treaty of Oliva, as the balance of power had been restored.