As the Soviets advanced into Bulgaria, the Red Army was also engaged in savage fighting on the Transylvanian passes in the Carpathian mountains leading into Hungary, but few doubted that it would be only a matter of time before the Soviets entered the Hungarian plain. On 21 September 1944, the Red Army took Arad, a Transylvanian city that was shortly occupied by the Hungarians, and panic broke out in Budapest. On 24 September 1944, the Regent of Hungary, Admiral Miklós Horthy, decided to open secret talks for an armistice with the Soviet Union, which he had resisted doing until then, sending Stalin a letter claiming that he was misinformed about the incident which led Hungary to become belligerent with the Soviet Union in 1941, and now accepted that the Soviets did not bomb the Hungarian town of Kassa. Like King Michael, Admiral Horthy hoped that signing an armistice now might save Hungary from a Communist regime, and furthermore he wanted to keep the part of Transylvania that Hungary had received under the Second Vienna Award of 1940. On 6 October 1944, the Battle of Debrecen began as the Red Army broke out onto the Hungarian plain. The Red Army captured and then lost Debrecen, through the three Soviet corps that had been encircled by the German counterattack were able to escape. The Soviet drive to Budapest had been halted for the moment, but it was assumed that the Red Army would resume its advance.