Tolui was less active than his elder brothers Jochi, Chagatai, and Ögedei during their father's rise to power, but once he reached adulthood he was considered the finest warrior of the four.

After the fall of the cities of Transoxiana in 1220, Genghis dispatched Tolui early the following year to subjugate the region of Khorasan, which had begun to cause trouble for the Mongol armies.

Medieval chroniclers attributed more than three million deaths to the massacres he ordered at Nishapur and Merv; while these figures are considered exaggerated by modern historians, they are evidence of the abnormal brutality of Tolui's campaign.

The year of Tolui's birth is disputed; while the historian Christopher Atwood believes he was born in 1191 or 1192,[2] the sinologists Frederick W. Mote and Paul Ratchnevsky placed the date in the late 1180s.

[6] The historian Isenbike Togan has speculated that "Tolui" was a title which Genghis intended to replace the pre-imperial epithet otchigin, traditionally given to the youngest son.

By this point, Jebe and Subutai had moved into western Iran, and the cities which had earlier submitted to them in the Khorasan region had become bolder; Genghis Khan's son-in law Toquchar was killed by a nascent rebellion at Nishapur in November 1220.

[18] After capturing Balkh in early 1221 and while continuing to besiege Taliqan, Genghis dispatched Tolui to Khorasan to make sure that no opposition remained in the extensive and wealthy region.

[19] Tolui's army was composed of a tenth of the Mongol invasion force augmented by Khwarazmian conscripts; the historian Carl Sverdrup estimates its size at around 7,000 men.

Having been subjected to a general assault on the seventh day, the townspeople, who twice attempted a sortie to no effect, lost the will to resist and surrendered to the Mongols, who promised to treat them fairly.

[23] Tolui, however, reneged on this guarantee, and ordered that the entire population be driven out on the plain and put to the sword, excluding a small number of artisans and children.

The early 20th-century historian Vasily Bartold, citing a local history from the 1400s, stated that none of the inhabitants were killed with the exception of the garrison; meanwhile, the chronicler Minhaj-i Siraj Juzjani, who fought the Mongols nearby, recorded that after an eight-month siege, the city was taken and its population slaughtered.

[33] The population subsequently rebelled and were besieged for months by the Mongol general Eljigidei, who was said to have killed between 1,600,000 and 2,400,000 people during his sack of the town, in a massacre lasting seven days in June 1222.

[37] The historian Michal Biran has suggested that the speed with which the Mongols brought the pragmatically brutal warfare of East Asia into the less ruthless Muslim world was a factor in this cultural shock.

This was due to Jochi's preference for remaining in and growing his own appanage—his actions during the Siege of Gurganj, where his reluctance to destroy a wealthy city that would become part of his territory eventually led to his failure to give Genghis the khan's share of the booty, exacerbated the tensions.

[49] Chagatai's attitude towards Jochi's possible succession—he had termed his elder brother "a Merkit bastard" and had brawled with him in front of their father—led Genghis to view him as uncompromising, arrogant, and narrow-minded, despite his great knowledge of Mongol legal customs.

As the youngest son, Tolui served as regent and administered the empire; possibly drawing upon previous traditions, he established a precedent for what to do after a khan's death.

These included the halting of all offensive military actions involving Mongol troops, the establishment of a lengthy mourning period, which the regent would oversee, and the holding of a kurultai which would nominate successors and select them.

[58] With the Tongguan Pass securely held by the Jin and the Mongol army suffering from famine in the depleted Shaanxi province, the brothers withdrew to Inner Mongolia to plan.

They decided to adopt one of their father's ideas: in a massive pincer movement, Tolui, accompanied by Subutai and Shigi Qutuqu, would bypass Tongguan by traversing Song territory to the south of Shaanxi, while Ogedei marched towards the Jin capital Kaifeng along the Yellow River.

[59] This risky strategy paid off—although Tolui's men allegedly suffered such deprivations they resorted to cannibalism—he successfully managed to gather provisions from untouched Song lands, cross back into the Jin province of Henan, and engage the enemy at Mount Sangfeng on 9 February 1232.

[64] Atwood has theorised that these suspicions were the intended aim of the Secret Historian, writing under the patronage of Tolui's descendants wanting to subtly discredit their Ögedeid rivals.

[65] He proposes that the prosaic explanation of a death from alcoholism, as recorded by Juvayni, was the most likely; the official line was nevertheless propagated by Sorghaghtani to cement her family's position at the top of the Mongol court.

[66] Sorghaghtani inherited Tolui's property after his death on Ögedei's command; with the backing of his vast estates in Mongolia, she became one of the most respected and powerful figures in the empire.