First of all, let's introduce what is 'wheel chop'', the so-called wheel chop refers to the Mongol iron horse led by Genghis Khan after each victory in war, a cruel method of killing prisoners, the wheel in the wheel chop is a division standard, all men who are taller than the wheel are killed, and women and young men who are lower than the wheel can save their lives.
The reason why the Mongol army carried out such a cruel wheel chop mainly had the following two considerations.
First, deter the enemy.
The origin of the Wheel Chop by the Mongol army was a campaign of extermination by Genghis Khan against the Tatars, who had killed Genghis Khan's father.
In this war, Genghis Khan won the final victory, and after the victory of the war, Genghis Khan executed all adult men of the Tatar tribe using the standard of wheel chopping.
The real origin of the wheel chopper actually comes from the Jin Kingdom, which once treated the Mongols in the process of its rise, called "Reducing Ding", and the Jin State would send a large army to the Mongolian steppe every year at that time, kill some men above the wheel, and rob slaves, women, children, livestock, and property.
Later, when Genghis Khan led the Mongol iron horse to sweep the world, the Mongol warriors were not heavenly gods, and there would be a large number of casualties, in order to preserve the living force, the soldiers who yielded without fighting, in many cases needed to establish their might, needed to kill chickens and monkeys, let other enemies be afraid, did not dare to harass their rear when the Mongols were fighting with other tribes, and forced them not to make senseless resistance and surrender as soon as possible when facing the Mongolian iron horse.
It has to be said that this policy has also played a certain effect, it is precisely because in the early days of Genghis Khan to promote this wheel chop, in the later period of conquest of the four sides, many small countries, small tribes after weighing the balance of strength between the two sides, are directly choose to surrender and submit, and the Mongol army for such opponents are also huairou, and will not take wheel chops, but only slaughter some stubborn tribes, further deter future opponents. This is also the "benefit" of implementing wheel chopping.
Second, the question of where the captives went.
Unlike our kind of agrarian civilization, nomadic peoples do not have fixed gathering places, and they mainly live through animal husbandry, and for the problem of prisoner handling after each war in the early stage, it is difficult for the Mongol army led by Genghis Khan to directly integrate their young adults into their own troops, and they must consider their loyalty.
As opponents of the same nomads, they are all natural cavalry, and the possession of horses by captives without loyalty means that there are convenient conditions for escape.
If the opponents are resolute in their will to resist, often harbor a disobedient heart, and have the traditional habit of rebelling and fleeing at every turn, then instead of leaving them to bring hidden worries to the future, it is better to cut the grass and remove the roots.
The iron horses of the Mongols lacked the traditional habit of killing women, and the women of the steppes were always valuable spoils of war, which could serve as a gift for passing on the generations and breeding the population of their own tribes.
Why leave a certain number of young children behind?
That's because they don't have the ability to pose a threat in the short term, so it doesn't hurt to leave them behind, and it is of course best to be able to brainwash them as a supplement to their combat strength when they are a little older, and if they have hatred in their hearts, they can also use it as a whetstone for their own tribal training in the near adulthood to train their newly grown soldiers.
In summary, the Mongol army's implementation of wheel chopping was not a consistent habit, mainly occurred in the early stages of expansion, and the reason was also because of the need for blood to forge the Mongol prestige, destroy the opponent's courage to resist, and also solve the problem of not being able to properly place prisoners.
By the end of the conquest, the Mongols had already established their prestige, so there was no need to carry out such cruel massacres, but to put these adult men who had lost the courage to resist into production and construction, and to grab greater benefits.