Regierung Dönitz
From his resignation as minister until 30 March 1944, he continued to serve Croatia in a diplomatic forum as ambassador to Hungary in Budapest. After that he became ambassador to the Third Reich in Berlin. In July 1944 he was named State Minister, but he stayed on as an ambassador in Berlin. Košak was a close associate of Lorković and was one of his best men, alongside Ante Vokić, at Lorković's wedding to Nada von Ghyczy in August 1944. Ante Pavelić intended to relieve him of all duties because he harbored suspicions that Košak was involved in the Lorković-Vokić coup, a coup in which Lorković and Vokić, along with many other influential politicians and military officers, intended to change side and join the Allies; the coup ended in failure. Košak was protected by Siegfried Kasche, German ambassador to Croatia, and was able to retain his title as ambassador to Germany. Following Pavelić's advice, Košak remained in Berlin after Hitler's death, intending to stay with the short-lived Flensburg Government. He was arrested by the British in Flensburg in May 1945. In February 1946 he was extradited to Yugoslavia and sentenced to death on 6 June 1947 for treason and war crimes; he was executed 12 days later, on 18 June.