"In AD 642, Alexandria was captured by the Muslim army of 'Amr ibn al-'As. Several later Arabic sources describe the library's destruction by the order of Caliph Omar. Bar-Hebraeus, writing in the thirteenth century, quotes Omar as saying to Yaḥyā al-Naḥwī: "If those books are in agreement with the Quran, we have no need of them; and if these are opposed to the Quran, destroy them." Later scholars, including Father Eusèbe Renaudot in 1793, are skeptical of these stories, given the range of time that had passed before they were written down and the political motivations of the various writers"
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@nygma
Looks like there is some nuance to this:
"In AD 642, Alexandria was captured by the Muslim army of 'Amr ibn al-'As. Several later Arabic sources describe the library's destruction by the order of Caliph Omar. Bar-Hebraeus, writing in the thirteenth century, quotes Omar as saying to Yaḥyā al-Naḥwī: "If those books are in agreement with the Quran, we have no need of them; and if these are opposed to the Quran, destroy them." Later scholars, including Father Eusèbe Renaudot in 1793, are skeptical of these stories, given the range of time that had passed before they were written down and the political motivations of the various writers"
From: https://en.wikipedia.org/wiki/Library_of_Alexandria
At this time also quite some new libraries got build, so a bit vague whether it was the same one the Julius damaged and does that matter...