sentences:  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.Charles's retreat from Jarosław to Warsaw almost ended with disaster, for he returned with the fragments of his host, for his army was trapped by the Polish-Lithuanian armies – amidst three converging armies, in a marshy forest region intersected in every direction by well-guarded rivers – is considered one of his most brilliant achievements.But on 21 June 1656 the Poles retook Warsaw, and four days later Charles was obliged to purchase the assistance of Frederick William I, by the treaty of Marienburg (23 June 1656).