The Royal Air Force carried out two bombing raids on Sofia, the capital of Bulgaria, in retaliation for the attacks on Yugoslavia, which surrendered on 17 April. The senior Luftwaffe officer responsible for the bombing, Generaloberst Alexander Löhr, was captured by the Yugoslavs at the end of the war and was tried and executed for war crimes, in part for his involvement in the bombing of Belgrade. Kren was arrested in 1947 on unrelated charges of war crimes stemming from his subsequent service as the head of the Air Force of the Independent State of Croatia.