They were overpowered and the advance of Tito 's Partisan forces , joined by the Soviet Red Army , caused a mass retreat of the Ustaše towards Austria and effectively an end to the Independent State of Croatia .
They were soon overpowered and the Independent State of Croatia ( NDH ) effectively ceased to exist in May 1945 , near the end of the war . The advance of Tito 's partisan forces , joined by the Soviet Red Army , caused mass retreat of the Ustaše towards Austria .