During the rule of Zabel and Het'um, the Mongols under Genghis Khan and his successor Ögedei Khan rapidly expanded from Central Asia and reached the Middle East, conquering Mesopotamia and Syria in their advance towards Egypt. On June 26, 1243, they secured a decisive victory at Köse Dağ against the Seljuk Turks. The Mongol conquest was disastrous for Greater Armenia, but not Cilicia, as Het'um preemptively chose to cooperate with the Mongols. He sent his brother Smbat to the Mongol court of Karakorum in 1247 to negotiate an alliance. He returned in 1250 with an agreement guaranteeing the integrity of Cilicia, as well as the promise of Mongol aid to recapture forts seized by the Seljuks. Despite his sometimes-burdensome military commitments to the Mongols, Het’um had the financial resources and political autonomy to build new and impressive fortifications, such as the castle at Tamrut. In 1253, Het'um himself visited the new Mongol ruler Möngke Khan at Karakorum. He was received with great honors and promised freedom from taxation of the Armenian churches and monasteries located in Mongol territory. Both during his trip to the Mongol court and in his 1256 return to Cilicia, he passed through Greater Armenia. On his return voyage, he remained much longer, receiving visits from local princes, bishops, and abbots. Het'um and his forces fought under the Mongol banner of Hulagu in the conquest of Muslim Syria and the capture of Aleppo and Damascus from 1259 to 1260. According to Arab historians, during Hulagu's conquest of Aleppo, Het'um and his forces were responsible for a massacre and arsons in the main mosque and in the neighboring quarters and souks.