The bombing of Kassa took place on 26 June 1941, when still unidentified aircraft conducted an airstrike on the city of Kassa, then 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.
Attack On 22 June 1941, Germany invaded the Soviet Union. Unlike Finland and Romania, Hungary was not asked to take part in Operation Operation Barbarossa and was allowed to remain neutral. Adolf Hitler did not want Hungary to join Operation Barbarossa in part because of Hungary's backwardness led him to believe that the Honved (Royal Hungarian Army) would be a net liability rather than an asset on the Eastern Front and in part because Hungary was a troublesome ally that kept making territorial claims against other German allies such as Romania and Slovakia. Hitler believed that if Hungary did take part in the war on the Eastern Front, they would demand to annex southern Transylvania and all of Slovakia as the reward. The most committed voice in Budapest urging Hungary to take part in Operation Barbarossa was General Henrik Werth, the chief of the Hungarian General Staff, who was strongly supported by the Defense minister, General Károly Bartha. Werth had already in May 1941 had the General Staff draw up plans for "offensive-minded defense" against the Soviet Union. Werth's primary reason for wanting to join the war was his belief expressed in a memo that he wrote that Hitler would reward Hungary for being "loyal" to Germany and that Hungary would "certainly recover the entire territory of historical Hungary" (i.e. the all the lands lost under the treaty of Trianon) by being "loyal".
On 23 June 1941, the Soviet Foreign Commissar Vyacheslav Molotov had a meeting with József Kristóffy, the Hungarian minister-plenipotentiary in Moscow, to tell him that the Soviet Union "...has no demands or hostile intent towards Hungary, has had no objection to Hungarian demands being realized at Romania’s expense, and in the future will have no objection in this regard either." Essentially, Molotov was saying that the Soviet Union would support Hungary's claim to southern Transylvania, which had remained Romanian under the Second Vienna Award, in exchange for Hungary remaining neutral. The fact that Romania under Ion Antonescu had joined Operation Barbarossa right from the beginning made Soviet diplomacy very hostile towards Romania after 22 June 1941. On the same day, the prime minister László Bárdossy called for a meeting of the council of ministers. At that meeting, Bartha advocated declaring war on the Soviet Union and told the council of ministers: "I think that having defeated the Poles in three weeks, having finished off the French in about as much time, and having beaten the Yugoslav Army in twelve days and occupied the entire Balkans in three weeks, in six weeks the Germans will be in Moscow and completely defeat Russia". The picture painted by Bartha and Werth was that entering the war would be a low risk endeavour as they stated that the war would not last long and that Hungarian losses would be extremely low. By contrast, Werth and Bartha argued that the rewards for entering the war would very great as they predicated that Hitler would reward Hungary for declaring war on the Soviet Union by restoring all of the Lands of the Crown of St. István and all of the terrority Hungary had lost under the Treaty of Trianon would be returned. Bárdossy rejected Bartha's advice to enter the war, but he did decide to break diplomatic relations with the Soviet Union. With the exception of the Interior Minister Ferenc Keresztes-Fischer, every minister supported Bárdossy's decision. Bárdossy did not inform the council of Kristóffy's telegram saying that the Soviet Union would support Hungary's claims against Romania in exchange for staying neutral, which could be used as a reason for continuing to be neutral.
On 26 June 1941, four days after Germany attacked the Soviet Union in violation of the Molotov–Ribbentrop non-aggression treaty as a part of Operation Barbarossa, three unidentified planes bombed the city, killing and wounding over a dozen people and causing minor material damage. Numerous buildings were hit, including the local post and telegraph office. The bombers seemed to have been aiming at the local artillery barracks for the Honved, but most of the bombs hit the post office and the houses around the barracks. By the time the Royal Hungarian Air Force had been able to send up some fighters, the bombers had already disappeared into the clouds. The news of the bombing reached Budapest at 1: 45 pm along with reports that some of the bomb fragments had writing in Russian. In the "intelligence register" of the General Staff concerning the bombing was added the unsigned statement that was in a different handwriting than the original statement that said: "Accordingly to a report coming in subsequently the attacking planes were Russian planes painted yellow". Upon receiving the news, Werth wasted no time in heading over to the Royal Castle along with Bartha to ask Horthy to declare war.
After received the reports of the bombing of Kassa on the morning of 26 June 1941, on the afternoon of the same day Bartha and Werth went to see the Regent of Hungary, Admiral Miklós Horthy at the Royal Castle at about 2: 00 pm. Both Bartha and Werth were categorical in insisting that the Soviet Union had bombed Kassa and demanded an immediate declaration of war, using highly emotional language as they claimed that Hungarian "honor" had been insulted and only a declaration of war could properly avenge this insult. The Hungarian historian Nandor Dreisziger wrote that Horthy was an impulsive man much given to rash decisions as he was prone to acting without fully thinking about the consequences of his actions, and that the regent needed someone like the former prime minister István Bethlen to advise him. Bethlen played the voice of reason and calm who persuaded Horthy not to go ahead with his more reckless plans to attack one of Hungary's neighbors (which was usually Romania as Horthy resented the loss of Transylvania very strongly), as he did numerous times during his time as prime minister between 1921-1931. The American historian Deborah Cornelius wrote about the regent's character that Horthy was "accustomed to command" owing to his lengthy service as an officer in the Imperial Austrian and Royal Hungarian Navy; as a sailor had visited every continent except for North America; noted that besides for his native Magyar, he was fluent in English, French, German, Italian and Serbo-Croat; was familiar with the court life as he served as the naval aide-de-camp to the King-Emperor Franz Josef; was dignified in his manner and been a very brave admiral to the point of recklessness in World War One as he seemingly had no fear of death. However she noted that: "Horthy was no diplomat and understood little about politics. Impulsive and prone to hasty decisions he was easily swayed by the last person he had spoken with. He followed the code of honor of a nineteenth century gentleman, was conservative, rabidly anti-Bolshevik and was overconfident in the abilities of his military leaders". On more negative note, American diplomats who served in Budapest usually characterized Horthy as "a honorable but stupid man" and "an old seadog on horseback", noting that Horthy did not really understand any of the issues facing his country and had fundamentally militaristic mindset, regarding all problems as being capable of being solved with a gun. Bethlen, Horthy's favorite adviser, was considered to be a restraining influence on the admiral during the interwar era as Bethlen was regarded as far more intelligent than Horthy and as someone who understood the problems facing Hungary.
The account of the bombing of Kassa given to Horthy by Werth and Bartha had pushed Horthy into a state of high excitement, but this time Bethlen was not present with his place occupied by Bárdossy who did nothing to argue against Bartha and Werth, and instead encouraged them. Horthy was greatly angered by the news of the bombing, which he saw as an act of unprovoked Soviet aggression and seems to have believed the claims of Bartha and Werth that the bombing was the prelude to a Soviet invasion of Hungary. Dreisziger argued that if Bethlen still been serving as prime minister in 1941 instead of Bárdossy, he might had been able to talk Horthy out of going to war against the Soviet Union. Bethlen had in fact written Horthy a letter advising him against declaring war on the Soviet Union, saying the bombing of Kassa was probably an accident, which Horthy did not read until after Bárdossy declared war. Bethlen wrote that he did not share the blind faith in a Nazi "final victory" that other Hungarian decision-makers that Bárdossy, Werth and Bartha had, and wrote that it was not inevitable that Germany would defeat the Soviet Union. Bethlen also added that based on his sources that the Honved was not ready for a major war and predicated Hungarian losses on the Eastern Front would be very heavy. Finally, Bethlen predicated that the war against Soviet Union would be unpopular and Hungary had no duty to defeat the Soviet Union "by sacrificing the flower of her people". In 1944, Bethlen wrote about the decision to declare war on the Soviet Union "the gullible Hungarian public was made to believe this by propaganda meant to trick us into the war by painting the bogey of Bolshevism on the wall". Horthy gave orders that the Royal Hungarian Air Force should start bombing targets in the Soviet Union immediately while telling Bárdossy to issue a declaration of war. Horthy was later to claim that he did not want to declare war and that Bárdossy exceeded his authority in declaring war as he maintained that just merely wanted to strike back via air strikes on Soviet targets to avenge the bombing of Kassa. The Royal Hungarian Air Force bombed the Soviet town of Stanyslaviv (modern Ivano-Frankivsk, Ukraine) at dawn on 27 June, a few hours before Bárdossy declared war.