It seems to have been the practice of the tribal Mongols to fight only when their horses were well fed. The Secret History tells of the consternation of Genghis’ men when they learned of an impending Naiman attack while they were out hunting. ‘Many of them said: “Our geldings are lean, what can we do now?”’ Clearly, horses which were in good enough condition for the hunt were not necessarily considered fit for war. But the quantities of grass needed to keep an army’s mounts in prime condition were enormous. This question has been studied by several modern scholars in pursuit of a lively debate about the role of logistics in setting limits to the Mongol conquests. Morgan has been among those who argue that it was shortage of pasture rather than the military resistance encountered that prevented the Mongols overrunning Syria, and the same argument has been applied to Subotei and Jebei’s Black Sea campaign, and to Genghis’ own abortive incursion into India. Professor Smith has produced some detailed calculations in support of this thesis. Starting with an estimate for a large Mongol army of 60,000 men and allowing five horses per man, it would have been necessary to find pasture for 300,000 horses. This does not allow for the herds of sheep and goats that also accompanied Mongol armies, of which the fourteenth-century Mamluk source al-Umari says that there could be as many as thirty per man. These would eventually be slaughtered and eaten during the course of a campaign, but without its horses a Mongol army was effectively helpless. Smith calculates that each horse would have required on average 9.33lbs of grass a day, which - taking the productivity of good Central Asian pasture land as 534lbs per acre per year - would mean that 300,000 horses would consume the grass over an area of about eight square miles every day. Water could also be a problem; assuming that a horse needs five gallons a day, of which half might be obtained from fresh grass, the theoretical 60,000-man army would consume at least three-quarters of a million gallons per day. A major river could cope with this demand, but the flow of most Central Asian and Middle Eastern rivers varies dramatically with the seasons, and would probably fall to inadequate levels during the summer and autumn. On this basis it would clearly be impossible for a Mongol army to remain very long in one place, even in well-watered grazing country, while the lower productivity of more arid regions might prevent campaigning by large armies altogether.
It is certainly important to bear these constraints in mind when considering the movements of Mongol armies, and especially those inexplicable withdrawals which often confused contemporaries. For example, the Egyptians appear to have thought that Hulegu’s withdrawal from Baghdad to Azerbaijan in 1261 was permanent, encouraging their puppet Caliph to risk a counter-attack, when in fact he was simply migrating in search of grazing just as a Mongolian community would do in peacetime. Marco Polo, writing forty years later, says that the Mongols in the Middle East were still following the same route, concentrating in Azerbaijan for the summer grazing, then moving to the warmer lowlands of Iraq in winter. In the 1250s the Armenian King Haithon went so far as to reassure the Crusaders in Palestine that the Mongols were no threat to them as they had only entered the region in search of grass for their horses. But Amitai-Preiss has argued convincingly that the problem of shortage of grazing has been exaggerated. One factor is that the Mongol expeditionary forces were often smaller than their enemies believed. We have seen that Genghis’ entire army at its peak numbered around 129,000 men, and although non-Mongol auxiliaries are not included in this total, these units would not generally have had the great horse herds that consumed the bulk of the supplies. Most campaigns were probably conducted with a fraction of this total, and even where very large armies were required to cross unproductive regions - as during the crossing of the Gobi in 1211 - they could have moved in separate columns, only concentrating when they reached more fertile country where opposition would be expected. The number of horses accompanying each trooper is also uncertain, and may well have varied according to the supply situation. One version of Marco Polo’s Travels claims that there were as many as eighteen per man, but Sinor favours an average of three or four. More significantly, the conquered areas could usually be counted on to supply other fodder than grass. Genghis, when he entered Bokhara, scandalised the religious community by ordering the receptacles in the great mosque to be cleared of their sacred books and filled with grain for his horses. Amitai-Preiss cites the example of Aleppo in 1281, whose inhabitants fled on the approach of a Mongol column, and ‘abandoned crops, granaries and foodstuffs’, and also points out that the Mongols would have had no reservations about letting their beasts graze in standing crops, or encroach on the pastures of local nomads. The same writer goes on to estimate that in the early twentieth century northern Syria was home to around 80,000 Bedouin nomads, which implies that a force of that size should have been able to support itself in a similar area of rather poor pasture, at least for the duration of a campaign.
At times it may nevertheless have been necessary for Mongol armies to employ emergency measures to solve their supply problems. Horses will in fact eat a wide variety of foods if necessary, including tree branches and even meat, and slaughtering some of the remounts would certainly have been preferable to seeing the entire herd starve. John de Plane Carpini and Marco Polo both report that the men themselves could subsist for up to ten days without having to stop to cook food. They survived mainly on dried milk, which was mixed with water and reconstituted into a liquid as they rode by the constant motion. They would supplement this with blood drawn from the veins of their horses, and occasionally with meat boiled in the stomach of the animal it came from. Some hostile sources accused the Mongols of resorting to even more drastic expedients. Matthew Paris, writing long afterwards about the invasion of Europe in 1241, repeated rumours that they were cannibals, who feasted on human corpses ‘as if they were bread’, and saved the most succulent young women for their officers. Carpini relates an equally unlikely tale that in 1214, during the invasion of north China, Genghis’ armies were so short of food that he had to order them to kill and eat one in every ten of their own number. Evidently the atmosphere of terror surrounding the Mongols was such that there were people prepared to believe them capable of anything.
As far as other military supplies were concerned, we have evidence for some sort of centralised network which could supply the troops with equipment for particular conditions likely to be encountered on campaign. For example, according to the Secret History Dorbei Doqshin’s men, who had been ordered to cut their way through the forest to surprise the Tumads during the campaign of 1216, took with them ‘axes, adzes, saws, chisels and other tools’. And Rashid ud-Din says that the men serving in Hsi Hsia in the unusually cold winter of 1225 to 1226 were provided with special sheepskin coats, and their horses with felt coverings. Unfortunately the details of the sophisticated supply department which must have been necessary to procure such items in the necessary quantities and ship them to where they were needed have not been recorded.