From the unpromising and desperately unforgiving background that forged his iron will and boundless ambition, Temujin (as Genghis Khan was named at birth) rose to build an empire that was to range from Korea and China, through Afghanistan, Persia and Iraq and eventually to Hungary and Russia, constituting the largest contiguous land imperium in history. His was an extraordinary, epic story and Frank McLynn does it full justice in a vivid, page-turning biography.

The author portrays well the extreme hardship of the nomadic life for Genghis as boy and man on the arid Mongolian steppe, where temperatures range between 100 degrees Fahrenheit and minus 43, and where ‘one can be hit simultaneously by winds from the Siberian tundra and desert storms from the Gobi’. (Readers of Tim Cope’s excellent 2011 book On the Trail of Genghis Khan will know that even for the 21st-century traveller on horseback the region is dangerous, arduous and topographically inhospitable.)

In an environment that bred hard men, Genghis was the hardest of them all. Born in 1162 (according to McLynn; other estimates vary from 1155 to 1167), by the age of 14 he had killed his half-brother (and potential rival) in an argument over a fish and had seized back his family’s horses, stolen in a raid. He married at 16, and when a competing clan abducted and impregnated his wife Borte he assembled a large army to rescue her. In 1206 he survived a poisoned arrow in his neck, and as reward for a brutally effective military career, a noble council (quriltai) of the Mongolian clans proclaimed Temujin their leader, or ‘Genghis [Chinggis] Khan’ — often translated as ‘Ruler of the Universe’. But at that point he was just warming up.

He reformed his army, the instrument of conquest, along Manchurian lines in decimal units: ten in a platoon, 100 in a company, 1,000 in a brigade and 10,000 in a division. Their pay was plunder. The wily Genghis also created a 10,000-strong imperial guard, making the sons of his generals officers in order to guarantee ‘good behaviour’. He unleashed this vast army of over 100,000 across Asia.

McLynn has subtitled his book ‘The Man Who Conquered the World’, but he might have added ‘and Slaughtered Half of It’. First Genghis subjugated — later all but annihilated — the Tanguts of north-western China, before invading China’s powerful Jin empire in 1211. ‘Like a shark, the Mongol empire had to be in continuous forward motion’ to sustain itself. By 1213 he was in Peking. The image of Mongolian warriors as fierce horsemen sweeping across the steppe is accurate, but incomplete. When confronted by the truly formidable defences of Peking, Genghis demonstrated great patience and resolve, starving the city into submission in 1215. The inevitable resulting sack ‘was one of the most seismic and traumatic events in Chinese history’.

From there his armies moved west and targeted Persia in 1219, where the Sultan had, in an act of extreme foolhardiness, deliberately provoked Genghis by shaving off the beards of two of his ambassadors and killing a third. Samarkand, that glorious city on the Silk Road, fell in 1220, despite the defenders’ super-weapon of two dozen war elephants. McLynn dismisses the oft-quoted figure of 50,000 killed there in a single day (note the limited time span), but admits ‘it is clear that the death toll was terrific and unacceptable’.

Worse was to come in 1221 — ‘a year to live in infamy’. While Genghis’s other armies had been busy in the east, threatening Tbilisi in Georgia and terrifying the Christian world, Tolui, one of Genghis’s equally reprehensible sons, took Merv (in modern-day Turkmenistan), one of the largest cities in the world. Promised safety, the citizens surrendered and emerged from behind their walls. Tolui ‘surveyed the masses dolefully gathered with their possessions, mounted a golden chair and ordered mass executions to commence’. They took four days and nights to complete. Genghis’s rotten fruit did not fall far from the tree.

Terror — and the certainty of its visitation — was a major weapon in Genghis’s arsenal: decapitated women, children and even cats and dogs were reputedly displayed. But while the butchery was indeed immense, it is worth questioning its extent on occasion: a depopulated city had little economic value, and imported colonisers could make up only so much of the shortfall.

McLynn is always careful not to be caught up in the hyperbole of mortality figures, settling on the ‘convincing’, but still sobering, total of 37.5 million deaths attributed to Genghis’s wars. He is right to stress the unprecedented scale of the carnage the genocidal Genghis wrought on his enemies, but perhaps is a little lenient in judging that ‘he exceeded in degree but not in kind the other killers of his age’. Numbers do count.

Genghis ended his blood-soaked career crushing a rebellion in 1226 with yet another trademark massacre. He died the following year — as his enemies pronounced him, ‘Accursed of God’.