By chance, the times and conditions were just right for conquest. A little later, and gunpowder would have stopped them dead. And cavalry was the perfect force, because, unlike an army of foot-soldiers, they did not need lines of supplies. They had their fuel with them, in the form of grass. Every man had spare mounts, and a carpet of fuel, in the form of the Asian grasslands, invited them westward, all the way to Syria and the Russian steppes.  

Along the way, the Mongols also acquired an ideology. As animists, they believed in countless nature spirits — of mountains, springs, rivers, trees — all under the aegis of their highest deity, Tengri, the Blue Sky. This was the god not only of the Mongols but of many other Inner Asian groups as well. A relic of this belief is to be seen today in the piles of rocks and pebbles, known as ovoos, that cap almost any hill or mountain. Their litters of offerings — empty vodka bottles, outdated banknotes, bits of blue silk — make them look like little rubbish dumps. If you come to one, you circle it three times clockwise, and add something to the pile, in deference to Tengri. The word also refers to weather, with a pairing similar to our Heaven/the heavens.

Traditionally, Tengri (Blue Heaven) was a remote entity, whose will might be interpreted by shamans. But Genghis’s success suggested that he was especially favoured, an idea that strengthened as the empire grew. Genghis’s heir, his third son Ogedei, turned it into a justification for yet more conquests, asserting that the whole world, of which they had not an inkling, had in fact been granted to the Mongols, and that it was their destiny to make everyone aware of this astonishing fact. It was Ogedei who sent the Mongol armies yet further, to Baghdad, to the shores of the Mediterranean, to southern Russia, to Poland, and the inviting grasslands of Hungary.    

Why stop there? Leaders across Western Europe trembled, talked of Christian unity, and froze like deer caught in headlights. Luckily for them, in 1241, back in Mongolia, Ogedei died, and the Mongol armies pulled back to take part in the struggle for succession, leaving a wilderness of burned towns and rotting bodies. They never returned, perhaps because they had discovered that the Hungarian grasslands were not large enough to fuel such a vast army.

Meanwhile, southern China remained untaken. Following a nasty family feud, the succession settled on the line of Genghis’s youngest son, Tolui, who was married to one of the most extraordinary women of the age, Sorkaktani. She, like the empire, deserves to be better known. Tolui died, but she, brilliantly combining patience and influence, saw her four sons become masters of the Mongolian universe. One (Hulegu) ruled Persia, one (Mönkhe) took over Mongolia itself, a third (Ariq) remained a wild card claiming his own realm in the heart of Asia, and the fourth was Kublai, who was made viceroy of north China by his brother Mönkhe. The two of them set about the invasion of China, the first step being the taking of Yunnan, then independent. In 1259, during the campaign, Mönkhe died and Kublai declared himself head of the whole empire.