Hungary did not immediately participate in the invasion of the Soviet Union. The invasion began on 22 June 1941, but Hitler did not directly ask for Hungarian assistance. Nonetheless, many Hungarian officials argued for participation in the war in order to encourage Hitler not to favour Romania in the event of border revisions in Transylvania. On 26 June 1941, the Soviet air force bombed Košice (Kassa). Some speculation exists that this was a "false-flag" attack instigated by Germany (possibly in cooperation with Romania) to give Hungary a "casus belli" for joining Operation Barbarossa and the war, although it is plausible that Soviet bombers mistook Kassa for nearby Prešov in Slovakia. Hungary declared war against the Soviets on 27 June 1941.