Kren was named the commander of the ZNDH upon returning to Zagreb in mid-April 1941 and served in this capacity until September 1943, when he voluntarily retired.

In June 1944, Croatian leader Ante Pavelić ordered that Kren be returned to active service within the ZNDH and had him re-appointed as its commander.

Before the outbreak of World War II, he was promoted to captain in the Royal Yugoslav Army and served as a battalion commander at Zagreb Airport.

There, he was promoted to colonel and named commander of the Air Force of the Independent State of Croatia (Croatian: Zrakoplovstvo Nezavisne Države Hrvatske, ZNDH).

[c] In mid-1944, Kren served as a mediator between the ZNDH and Croatian politicians Mladen Lorković and Ante Vokić, the main conspirators behind the failed Lorković–Vokić plot.

As commander, Kren visited an academy for young ZNDH pilots in Italy and toured Bulgaria, Germany, and the Eastern Front.

During his stay in Germany, he negotiated the sale of German aircraft to the NDH and was granted access to several facilities that were used to manufacture the V-1 flying bomb and the V-2 rocket.

[6] In March 1947, British intelligence discovered that Kren was staying in Genoa under the protection of Monsignor Karlo Petranović and that he was planning to leave for Argentina under the alias Marko Rubini.

Kren was arrested by British authorities on 4 March 1947 while attempting to board a ship called Philippa, which regularly carried suspected Croatian war criminals along the Genoa–Buenos Aires route.