Soviet planes bomb Kassa, giving Hungary the impetus to declare war the next day
now Košice, Slovakia
The bombing of Kassa took place on 26 June 1941, when still unidentified aircraft conducted an airstrike on the city of Kassa, then a part of Hungary, today Košice in Slovakia. This attack became the pretext for the government of Hungary to declare war on the Soviet Union the next day, 27 June.
On 26 June 1941, four days after Germany attacked the Soviet Union in violation of the Molotov–Ribbentrop non-aggression treaty, three unidentified planes of apparently Soviet origin bombed the city, killing and wounding over a dozen people and causing minor material damage.
The true identity of the attacking nation has never been established.
The official explanation preferred by Soviet historians was the idea of a feigned attack by Germany to provoke Hungary into attacking the Soviet Union, employing Soviet planes captured on conquered airfields. Another possibility is that the Soviet bombers mistook Kassa for a nearby city in the First Slovak Republic, which was already at war with the Soviet Union. Colonel Ádám Krúdy, the commander of the Kassa military airfield, identified the attackers as German Heinkel He 111 bombers in his official report but was ordered to keep silent about it.
According to historians Iván Pataki, László Rozsos and Gyula Sárhidai, Soviet aircraft attacked the city by mistake while targeting a German radio station in the city of Prešov (Eperjes), Slovakia, about 30 kilometers north of Kassa. All three bombers aimed at the post office building which had a large radio antenna on its roof. After releasing a total of 29 bombs, one of the planes dropped a single bomb outside the city which did not explode; it was recovered and identified as a 105 kg Soviet bomb. According to this explanation, Krúdy misidentified three Soviet TB-3 bombers as German He 111s, but the suggestion that this identification was "hushed up" represented post-war Communist propaganda.
After the incident Hungary declared itself at war with the Soviet Union.
Sources: wikipedia.org, news.lv