The answer to this lies in a comment of Kirakos Ganjaketsis in his History Of The Armenians; where he refers to Baghdad as a Tachik city. Tachik or Tajik means city dweller
When the Mongols came for Baghdad, they sent a party of engineers to make the Tigris impassable to civilian refugees. This party – about 3,000 in total – were intercepted by a detachment of Caliph Al-Musta’sim’s army and killed. This was the caliph’s last victory.
The loss of these engineers was less than a pinprick to the Mongols. A Mongol army was by then a mobile nation. Their mounted warriors were feared, however their campaigns – which involved considerable amounts of rape, torture, looting and slaughter – attracted certain types of people. So a Mongol army might come to well over 100,000, attracted to which would be a considerable number of opportunists – mercenaries, outlaws and survivors – who became a vanguard. Add to that the normal camp followers, especially those who could make or repair anything to do with waging war; weapons, food foraging, bridges, siege towers – the supply chain was short – which meant that a Mongol army was like a nation of locusts on the move.
Shortly after, the city was surrounded and the usual Mongol terms were offered.
‘From the King of Kings of the East and West, the Great Khan. To Al-Musta’sim the Tachik (the honorific ‘caliph’ would be omitted, Tachik = city dweller), who fled to escape our swords. You should think of what happened to other countries and submit to us. You have heard how we have conquered a vast empire and have purified the earth of the disorders that tainted it. We have conquered vast areas, massacring all the people. You cannot escape from the terror of our armies. Where can you flee? What road will you use to escape us? Our horses are swift, our arrows sharp, our swords like thunderbolts, our hearts as hard as the mountains, our soldiers as numerous as the sand. Fortresses will not detain us, nor armies stop us. Your prayers to God will not avail against us. We are not moved by tears nor touched by lamentations. Only those who beg our protection will be safe. Hasten your reply before the fire of war is kindled. Resist and you will suffer the most terrible catastrophes. We will shatter your mosques and reveal the weakness of your God and then will kill your children and your old men together.’
The Mongols employed scribes to put a velvet over their iron fist. The caliph’s forces had beaten back the Mongols in 1238 and again in 1245. He was flush with success from driving off, or killing a band of engineers and replied with his best insults, proclaiming that he was lord of sea and land and had the banner of Mahmet,
“It is here and, if I touch it, you and the entire world will be destroyed. You are a dog, a Turk. Why should I pay taxes to you or obey you?” Kirakos Ganjaketsis: History Of The Armenians
The usual pleasantries over, the business of siege began.
The Mongols were cunning – renowned for savage destruction. They offered to let those who didn’t want to fight, to leave in peace. Many took up this offer. These gathered at the city walls, ready to be escorted safely away from the coming battle by the besieging Mongols. Party by party, they were escorted away from the doomed city, until that phase was done. Those who sought peace were escorted only far enough away from the city so that their screams couldn’t be heard as they were butchered.
Then the siege began in earnest and ended in the normal way; plunder, torture, rape, butchery and (I seem to remember) a vast pile of severed heads.
‘And the arrogant and fanatical kingdom of the Tachiks ended after a duration of six hundred and forty seven years.’ Kirakos Ganjaketsis: History Of The Armenians
Baghdad fell in 1258, it had had a population of around one million and was the last major city of the Tachiks to fall. The Tachik cities were those that made up Khwarezm and the rump of the Abassid caliphate – effectively Baghdad. The Mongols were nomads, they despised Tachiks. The thirteenth century historian, Juvaini, who served as an administrator in the Mongol army, reckoned that about 1% survived.
Slaughter left an empty land, but life still went on. The Mongols repopulated Khwarezm with Turks and other peoples from their vast empire.
Tachik (Tajik) became an ethonym. It (Tachik) also became a term of disrespect and an insult which stuck down the centuries; this last bit of Mongol mischief didn’t begin to unwind until Soviet times.
Who were the Khwarezmians?
They were city dwellers.
What was their population?
About 30 million – Khwarezm ruled the old Persian Empire and more; it was the eastern equivalent of the Roman Empire and unlike the Rome, it still flourished.
Where are their descendants?
Scattered in and around Tajikistan.