Tamerlane

Tamerlane, aka the Scourge of God (originally attributed to Attila the Hun and later proclaimed by Ghengis Khan), was not a man to be trifled with. He was the second great Khan, and sought to finish what Ghengis Khan had started. When his armies captured Baghdad in 1401, the massacred 20,000 inhabitants and reduced Baghdad to rubble. His soldiers were ordered to return with at least two severed heads. This kind of brutality was standard operating practice. In Tamerlane's own words, when describing his invasion of India in 1389 and right before the sack of Delhi, gives us an idea of the Mongol approach to conquest:

At this Court Amir Jahan Shah and Amir Sulaiman Shah, and other amirs of experience, brought to my notice that, from the time of entering Hindustan up to the present time, we had taken more than 100,000 infidels and Hindus prisoners, and that they were all in my camp. On the previous day, when the enemy’s forces made the attack upon us, the prisoners made signs of rejoicing, uttered imprecations against us, and were ready, as soon as they heard of the enemy’s success, to form themselves into a body, break their bonds, plunder our tents, and then to go and join the enemy, and so increase his numbers and strength. I asked their advice about the prisoners, and they said that on the great day of battle these 100,000 prisoners could not be left with the baggage, and that it would be entirely opposed to the rules of war to set these idolaters and foes of Islam at liberty. In fact, no other course remained but that of making them all food for the sword. When I heard these words I found them in accord with the rules of war, and I directly gave my command for the Tawachis to proclaim throughout the camp that every man who had infidel prisoners was to put them to death, and whoever neglected to do so should himself be executed and his property given to the informer. When this order became known to the ghazis of Islam, they drew their swords and put their prisoners to death. 100,000 infidels, impious idolaters, were on that day slain. Maulana Nasiru-d din ‘Umar, a counsellor and man of learning, who, in all his life, had never killed a sparrow, now, in execution of my order, slew with his sword fifteen idolatrous Hindus, who were his captives.

The Mongol Solution to counter-insurency; slaugter everyone.

The construction Bibi-Khanym Mosque was a direct result of Tamerlane's invasion of India. Tamerlane (also known as Timor) tells us how he did it:

I ordered that all the artisans and clever mechanics, who were masters of their respective crafts, should be picked out from among the prisoners [from the sack of Delhi] and set aside, and accordingly some thousands of craftsmen were selected to await my command. All these I distributed among the princes and amirs who were present, or who were engaged officially in other parts of my dominions. I had determined to build a Masjid-i jami in Samarkand, the seat of my empire, which should be without a rival in any country; so I ordered that all builders and stone-masons should be set apart for my own especial service.

Edgar Allen Poe immortalised Samarkand in his poem Tamerlane (first published Poe poem, btw):

Look 'round thee now on Samarcand! —
Is not she queen of Earth? her pride
Above all cities? in her hand
Their destinies? in all beside
Of glory which the world hath known
Stands she not nobly and alone?
Falling — her veriest stepping-stone
Shall form the pedestal of a throne —
And who her sovereign? Timour — he
Whom the astonished people saw
Striding o'er empires haughtily
A diadem'd outlaw —

Oddly enough, Tamerlane's justification wiping out Jats during the invasion of India (which built the great mosque), has a vaguely modern ring to it (bold added):

On the 8th of the month I marched from Ahruni, through the jungle to a village called Tohana. In answer to the inquiries I made about the inhabitants, I learned that they were a robust race, and were called Jats. They were Musulmans only in name and had not their equals in theft and highway robbery. They plundered caravans upon the road, and were a terror to Musulmans and travelers. They had now abandoned the village and had fled to the sugar-cane fields, the valleys, and the jungles. When these facts reached my ears I prepared a force which I placed under the direction of Tokal Bahadur, son of the Hindu Karkarra, and sent it against the Jats. They accordingly marched into the sugar-canes and jungles. I also sent Maulana Nasiru-d din in pursuit of them. When these forces overtook the Jats they put 200 to the sword and made the rest prisoners. A large stock of cattle was captured, and my soldiers returned to camp.

It was again brought to my knowledge that these turbulent Jats were as numerous as ants or locusts, and that no, traveler or merchant passed unscathed from their hands. They had now taken flight, and had gone into jungles and deserts hard to penetrate. A few of them had been killed, but it was my fixed determination to clear from thieves and robbers every country that I subdued, so that the servants of God, and Musulmans and travellers might be secure from their violence. My great object in invading Hindustan had been to wage a religious war against the infidel Hindus, and it now appeared to me that it was necessary for me to put down these Jats and to deliver travelers from their hands.