What really happened to Baghdad !

It wasn't
Al Alqamy ! - that was a myth they used to tell us in schools here in the Arab world
the myth states that Al Alqamy seduced the caliph to ignore military matters (which is false : the Abbasids were already in decline and were not able to pay their troops well). They tell us that Al Alqamy opened the doors (as simple as that !) because Hulagu khan promised him that he will be the future governor of Baghdad.
in realty the records don't mention Al Alqamy, & the person who ended up as governor of Baghdad was actually
Ala'iddin Ata-Malik Juvayni.
simply
1 - no one expected them
2 - the Abbasids were already weak
3 - the mongols used siege technology
no one wants to admit that they were beaten by an overwhelming force -
so thy just invent lies to justify their failure & ignore the reality of the situation - we always think we are superior & stronger while our foes were inferior, worthless, naked scum.
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every nation has its own version of the movie 300
Another myth was that the mongols were book destroyers and that they brought illiteracy to the region
most people think this is true unfortunately and we hear it in schools and sermons - mongol book hunt / burning or... whatever
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that they gathered all the books and through then into the river just to cross it so the river ran black from the ink of so many lost written treasures.

This story was stolen from a late Zoroastrian propaganda against the caliph Omar. they said that he burned their religious text & threw the rest into the rivers. He then burned the library of Ctesiphon and massacred all the scholars, which left them illiterate for the coming generations.

in reality the books all survived and were studied later on in the house of wisdom in Baghdad.
the library of Ctesiphon was still working and in use
until the Abbasid period where al the scholars moved to work in Baghdad.
the building later fell into ruin during the 10th century. No scholar was killed
why would anyone destroy something he/she new was economically important
so what really happened to the house of wisdom ? - it simply caught fire when the city was burned
You can conducted scorched earth policy to cut losses and subdue the rest of the world with fear.
but you cant massacre every-where all the time (I experimented that while playing Stainless steel )
- you will need subjects, workers and a population to sustain the land
they needed farmland and they needed urban centers and trade
- scorched earth policies were only conducted when needed and when they were expected to give the intended results
- after they established a firm grip on their newly conquered land massacres where not necessary anymore
The didn't destroy everything just for fun - if they did they would waste their effort rebuilding everything all the time
In fact the Islamic civilization survived the mongol conquests
both the
Mamluks and the
ottomans were powerful enough and art, literature, science and technology flourished again
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it fell later during the age of colonialism along with the New-World, Africa, Asia, Australasia and the Pacific