In the beginning of 1656 King John II Casimir returned from exile and the reorganised Polish army, increased in numbers. By this time Charles had discovered that he could more readily defeat the Poles than conquer Poland. What is described as his chief object, the conquest of Prussia, remained unaccomplished, and a new Swedish adversary arose in the elector of Brandenburg, Frederick William I, alarmed by the ambition of the Swedish king. Charles forced the elector, albeit at the point of the sword, to become his ally and vassal (Treaty of Königsberg, 17 January 1656); but the Polish national rising now imperatively demanded his presence in the south. For weeks he engaged in the pursuit of Polish divisions engaged in guerrilla tactics in the snow-covered plains of Poland, penetrating as far south as Jarosław in Ruthenian Voivodeship (województwo ruskie), by which time he had lost two-thirds of his 15,000 men army with no apparent result. In the meantime, the Russians signed a cease-fire with the Polish–Lithuanian Commonwealth (Treaty of Vilno of 1656 ) and then pursued a campaign in Livonia and laid siege to Riga, the second largest city in the Swedish Realm.