China unimpeded. The Song also lost the horse-producing regions which made their cavalry extremely inferior. The military technology of the Song included gunpowder weapons such as fire lances, cast-iron gunpowder bombs, and rockets were employed in large numbers. The Song government also created China's first standing navy. This military technology and prosperous economy were key for the Song army to fend off invaders who could not be bribed with "tribute payments," such as the Khitans and Jur'chens. Song forces held off Central Asian Mongol armies longer than did other settled peoples, until the fall of the Song in 1279. Yuan Founded by the Mongols who conquered Song China, the Yuan had the same military system as most nomadic peoples to China's north, focused mainly on nomadic cavalry, who were organized based on households and who were led by leaders appointed by the khan. The Mongol invasion started in earnest only when they acquired their first navy, mainly from Chinese Song defectors. Liu Cheng, a Chinese Song commander who defected to the Mongols, suggested a switch in tactics, and assisted the Mongols in building their own fleet. Many Chinese served in the Mongol navy and army and assisted them in their conquest of Song. However, in the conquest of China, the Mongols also adopted gunpowder weapons such as the thundercrash bomb and thousands of Chinese infantry and naval forces into the Mongol army. Another weapon adopted by the Mongols were Saracen counterweight trebuchets designed by Muslim engineers; these proved decisive in the Siege of Xiangyang, whose capture by the Mongols precipitated the beginning of the end for the Song dynasty. The Mongol military system began to collapse after the 14th century and by 1368 the Mongols was driven out by the Chinese Ming dynasty. The Mongols under Genghis Khan and Hulagu also brought Chinese artillery specialists within their armies who specialized in mangonels, to Persia. During the Mongol invasion of Iraq, 1,000 Chinese crossbowmen who utilized fire arrows participated in the invasion, along with the Mongol tribesmen. In 1258 the commander of the Mongol Hulagu Khan's forces besieging Baghdad was a Chinese General Guo Kan. The Chinese General Guo Kan was then made Governor of Baghdad by Hulagu, who also brought Chinese technicians specializing in hydraulics to engineer the Tigris–Euphrates basin irrigation systems. This resulted in the Middle East being permeated by major Chinese influence during Hulagu's reign. Many Han Chinese