army having maneuvered along the Jin rear, Ogedei was able to send reinforcements to Subutai, bringing the total Mongol strength to 50,000 men. After these reinforcements arrived and with the Jin army's food supplies severely depleted over the past three weeks, Subutai forced a battle on his terms and won a decisive victory at the Battle of Sanfengshan, capturing Wan Yen Heda and annihilating the main Jin army which had nowhere safe to retreat to. He then made forced marches and eliminated the other Jin armies holding the other fronts at the battles of Yangyi (24 February 1232), and T’iehling (1 March 1232). The heavily fortified city of Kaifeng required an eight month long siege. Subutai was forced to construct lines of circumvallation that had a perimeter stretching . Additionally, the Jin began to employ a cutting edge gunpowder weapon called "Thunder Crash Bombs", which made it very difficult for the Mongols to get close enough for more concentrated fire. After cutting off Kaifeng from any outside help, Subutai alternated intense bombardments using a mixture of Muslim trebuchets, mangonels, and captured gunpowder with periods of rest and plundering the countryside. The Mongol dispersion helped them avoid contagion by the plague that devastated Kaifeng, and after being stretched to the limit, the Emperor fled and the city surrendered. Subutai originally wanted to execute everyone in Kaifeng to punish them for their intransigence and extremely long resistance. However, Ögedei intervened and ordered Subutai to treat them fairly. With assistance from the Song Dynasty, the last Jin stronghold of Caizhou fell in 1234. However, it did not take the Song long to fall out with the Mongols. Two Song armies seized Kaifeng and Luoyang during the summer of 1234 during Subutai's absence. Subutai returned, destroyed the three Song armies by isolating and defeating them, and retook the cities. He then made a preemptive raid into Song territory to force them to adopt a defensive position. Though war had begun between the Mongols and Song, Subutai was recalled to the west. Still, Subutai's victories had taught the Song a lesson they would heed: no Song army would dare venture north to attack Mongol territory afterward. Conquest of Rus' (1236–1240) Ögedei decided to send a major part of the army into the western regions to finally crush the Wild Kipchaks and Bulgars. Subutai was tasked to direct the operations (under the overall command of prince