This was seen as a grave insult to the Khan himself, who considered ambassadors "as sacred and inviolable."
By 1219 AD, the Mongol Army reached the borders of the Khwarezmian Empire. The cities of Bukhara & Urgench and the capital Samarkand was destroyed.
Population of Khwarezmian Empire (1218) : 2.7 Million
Population of Khwarezmian Empire (1221) : 0.3 Million
But Iltutmish was too smart and knew that he could enrage the Mongols by helping the young prince.
The next move of Iltutmish saved the Indian Sub Continent.
The Mongols respected the decision of the Delhi Sultanate, and Subutai went to Europe and Genghis back to China. Thus the first wave of Mongol Horde was stopped at Indus.
Baghdad (The largest city on earth at that time) was wiped out in 1258, and its entire population of 1.1 million was massacred. It was the end of the Abbasid Caliphate.
For the first time in Muslim History, Islam had no Caliph.
At this time, Alauddin Khilji was successful in not just stopping the Mongols but also pushing them back.
In 1203, Khilji made a tower of 8000 - 60000 decapitated Mongol heads (sources vary) after repelling the Mongol invasion & cemented them on walls of Siri (Head) fortress.
It was said that for three months after Timur left, no living thing moved in Delhi
After the Mongols, came the plague 'Black Death'. Again India & Japan survived.