Ян II Казимир
Brandenburg-Prussia was obliged to give military aid to Poland against the Swedish Empire during the ongoing Second Northern War. Frederick William I had in Wehlau agreed to aid John II Casimir Vasa with 8,000 men, and both parties agreed on an "eternal alliance". In Bromberg, it was agreed that from his Prussian province, Frederick William I would dispatch 1,500 foot and 500 horse to join the army of the Polish king.