Soon he became president of the Pension Fund of Private Officers, after which he became Chief Director of Pohit, the largest industrial holding in Croatia at the time.

[1] On 1 July 1936, Košak joined the fledgling Ustaše[2] and participated in starting of newspaper Hrvatski narod (The Croatian People).

Two days later, he was named the personal proxy of the Minister of Armed Forces, Vojskovođa Slavko Kvaternik.

[1] From his resignation as minister until 30 March 1944, he continued to serve Croatia in a diplomatic forum as ambassador to Hungary in Budapest.

Following Pavelić's advice, Košak remained in Berlin after Hitler's death, intending to stay with the short-lived Flensburg Government.