"I will burn your city, your land, your self" - the sack of Baghdad in 1258 and the last of the Abbasid caliphs
"When I lead my army against Baghdad in anger, whether you hide in heaven or in earth, I will bring you down from the spinning spheres; I will toss you in the air like a lion. I will leave no one alive in your realm; I will burn your city, your land, your self. If you wish to spare yourself and your venerable family, give heed to my advice with the ear of intelligence. If you do not, you will see what God has willed." (Hülegü Khan in a letter to Al-Musta'sim, last Abbasid Caliph of Baghdad)
20 February 1258, 755 years ago, Al-Musta'sim Billah was executed by the Mongol conquerors. Since their leader Hülegü obviously had resentments to spill "royal blood" after the sack of the cultural centre of Islam and the death of up to a million people, Al-Musta'sim was rolled into a carpet and trampled to death by Mongol riders.
The utter destruction of Baghdad and the death of the last of the Abbasids who had ruled since the 8th century marks the end of the Islamic Golden Age and a blow from which the Arab-dominated Middle East never really recovered. The fertile crescent between the Tigris and the Euphrates became a semidesert when the thousands-of-years-old irrigation system was vandalised and not enough people left alive to repair it, the Tigris ran dark not only with blood but with the ink of books and scrolls from the great library, the "House of Wisdom" flung into it, monuments were burned to the ground and Hülegü finally had to move his army upwind because the stench of the ruined metropolis had become unbearable.
Nonetheless, the place managed to recuperate at least in terms of economy for the next century under the Mongol rulers of Iran until it was again totally destroyed by Timur ("Tamerlane") in 1401.