At the time, Baghdad was perhaps the greatest city in the world, in terms of population size, wealth, trade, and importance as a center for learning, thanks to the House of Wisdom. Its destruction was equivalent to the destruction of a London or New York City in modern times.
Hulagu Khan's army had begun a siege on 29 January, 1258. It would have been unnecessary, if the Caliph al-Musta'sim had agreed to pay tribute to the Mongols. For whatever reason, he chose to defy the invaders. al-Musta'sim could have called for reinforcements, but did not. He had, unfortunately, alienated the Mamluks, one of the few groups that would, a little after this event, actually stop the progress of the Mongols (which it happens I wrote about exactly nine years ago today).
Hulagu's siege engines and catapults subjected Baghdad's walls and inhabitants to an endless barrage that did not pause at nightfall. In a week's time they controlled a section of the wall, at which point Musta'sim tried to negotiate. His attempt was rejected. Several important men of the city tried to approach Hulagu themselves with offers of truce; they were all murdered. On 10 February, the city surrendered, hoping to avoid complete destruction.
Their hopes proved futile. The Mongols entered the city on the 13th and began the slaughter. Citizens trying to flee the city were cut down; women and children were not spared. Although descriptions of the destruction were no doubt exaggerated over time—especially to underscore the barbarous nature of the Mongols—it was horrific. The books from the House of Wisdom were thrown into the Tigris, as well as those of three dozen other public libraries. The philosophers and scientists who maintained the House were murdered, and the Tigris was said to run red with their blood. Every building and mosque was destroyed.
A few years later, in 1262, the Mongols boasted to Louis IX of France that they killed 2,000,000 that day. That number was probably twice the total population of Baghdad, but it is very likely that several tens of thousands were killed in the course of a few days. Hulagu had to move his camp upwind of the city because of the stench of corpses over the next several days.
I'll tell you more of the story, the fate of al-Musta'sim, and the aftermath, next time.