<P> The estimates for the sizes of the opposing armies are often in dispute . All contemporary Muslim historians claim that the Mongol army was larger, with several chroniclers providing the figures of 400,000 for the Shah (spread across the whole empire) and 600,000--700,000 for the Khan . 800,000 for Genghis was also thrown around, though with less frequency . Modern historians still debate to what degree these numbers reflected reality . David Morgan and Denis Sinor, among others, doubt the numbers are true in either absolute or relative terms, while John Mason Smith sees the numbers as accurate as for both armies . Sinor uses the figure of 400,000 for the Khwarezmians, but puts the Mongol force at 105,000--150,000 . Near - contemporary sources, such as Rashid Al - Din, state that the Mongols had 105,000 soldiers total (in the whole empire, not just on a campaign) in 1206, and 129,000 in 1227 . </P> <P> Carl Sverdrup, using a variety of sources and estimation methods, gives the number of 75,000 for the Mongol army . Sverdrup also estimates the Khwarezmian army at 40,000, 1 / 10 of the contemporary 400,000 figure, and states that all contemporary sources are in agreement that, if nothing else, the Mongol army was the larger of the two . He states that he came to 40,000 by first calculating the size of the Mongol army based on their historical records, and then assuming the Kwharezmian army was exaggerated by the chroniclers to about the same magnitude as the Mongol one . McLynn also says that 400,000 is a massive exaggeration, but considers 200,000 to be closer to the truth, and supports a Mongol army size of over 100,000 . Genghis brought along his most able generals, besides Muqali to aid him . Genghis also brought a large body of foreigners with him, primarily of Chinese origin . These foreigners were siege experts, bridge - building experts, doctors and a variety of specialty soldiers . </P> <P> During the invasion of Transoxania in 1219, along with the main Mongol force, Genghis Khan used a Chinese specialist catapult unit in battle; they were used again in 1220 in Transoxania . The Chinese may have used the catapults to hurl gunpowder bombs, since they already had them by this time . While Genghis Khan was conquering Transoxania and Persia, several Chinese who were familiar with gunpowder were serving with Genghis's army . Historians have suggested that the Mongol invasion had brought Chinese gunpowder weapons to Central Asia . One of these was the huochong, a Chinese mortar . </P> <P> In this invasion, the Khan first demonstrated the use of indirect attack that would become a hallmark of his later campaigns, and those of his sons and grandsons . The Khan divided his armies, and sent one force solely to find and execute the Shah--so that a ruler of an Empire as large as that of the Khan's, with a larger army, was forced to run for his life in his own country . The divided Mongol forces destroyed the Shah's forces piecemeal, and began the utter devastation of the country which would mark many of their later conquests . </P>

. in 1219 which mongol king attacked trans oxania