I watched a history document long ago so my memory may be off on it. So I looked it up to see where I was off. Here is where I wasn't as accurate. using the source
The Burning of the Library of Alexandria | eHISTORY
"Alexandria was founded in Egypt by Alexander the Great. His successor as Pharaoh, Ptolemy I Soter, founded the Museum (also called Museum of Alexandria, Greek Mouseion, “Seat of the Muses”) or Royal Library of Alexandria in 283 BC."
" It contained It has been estimated that at one time the Library of Alexandria held over half a million documents from Assyria, Greece, Persia,
Egypt, India and many other nations."
"The final individual to get blamed for the destruction is the Moslem Caliph Omar. In 640 AD the Moslems took the city of Alexandria. Upon learning of "a great library containing all the knowledge of the world" the conquering general supposedly asked Caliph Omar for instructions. The Caliph has been quoted as saying of the Library's holdings, "they will either contradict the Koran, in which case they are heresy, or they will agree with it, so they are superfluous." So, allegedly, all the texts were destroyed by using them as tinder for the bathhouses of the city. Even then it was said to have taken six months to burn all the documents. But these details, from the Caliph's quote to the incredulous six months it supposedly took to burn all the books, weren't written down until 300 years after the fact. These facts condemning Omar were written by Bishop Gregory Bar Hebræus, a Christian who spent a great deal of time writing about Moslem atrocities without much historical documentation."
After reading. There are 3 main stories about who burned the library. One involved Julius Caesar, The other was it was lost in a natural decline of the Roman Empire, and the last story is that the Moslems burned then the library because they believed it contradicted the Koran. Which is the same reason the Modern Taliban destroyed libraries. It was the third story that I had learned. I don't remember the original documentary that I watch but it was from a reliable source. My mind just didn't remember it accurately. Along with the accuracy of my initial thoughts about it. But after researching I now remember.
"I was thinking that Alexander the Great saved the knowledge because he created the library which allowed the knowledge to spread across cultures." Had it not been for the library much of the knowledge would have been lost. Here's a video of the 3 stories. around the 42:00 mark