The army complained of near friendly fire incidents while fighting through the city and smoke made life difficult for the German artillery spotters. Hitler, despite the complaints, ordered the bombing to continue. Richthofen's force also flew 450 sorties against Modlin Fortress, securing the town's surrender on 27 September after 318 tonnes of bombs had been dropped on it in two days. Warsaw surrendered soon afterwards, and the campaign was declared over after the Polish surrender on 6 October 1939.