On this date, September 17th, in 642, Omar (the second Caliph) conquered Alexandria, Egypt, and destroyed the Library of Alexandria, the largest in the world at the time.
Omar (or Umar) waged a great offensive war to increase the size of Dar al-Islam, and to forcibly convert non-Muslims to Muslim by the sword. He conquered Mesopotamia (Iraq) and Persia (Iran), and destroyed the Zoroastrian Sassanid empire. He also conquered Palestine (Israel), Syria, Egypt, and Algeria, thereby taking them from the Byzantine empire. It was also Omar who first decreed that non-Muslims were not to be allowed into the holy, holy cities of Mecca and Medina (the very heart of the Religion of Peace™) upon penalty of death.
During his conquest of Egypt his troops, under the command of Amr Ibn Al Ass, conquered the city of Alexandria (Al-Iskandariyya). The Library of Alexandria contained a million scrolls, and included works by Aristotle, Eratosthenes, Aeschylus, Sophocles, Euclid, Ptolemy, and Euripides. Al Ass sent a messenger to ask Omar what to do with the massive collection. Omar sent his reply:
“[…] concerning the books that you have mentioned, if they contain things in accordance to Allah’s Book [the Koran] then Allah’s Book is enough. And if they contain things that contradict what is found in Allah’s Book, then we do not need them.”
Either way, the books were deemed unimportant, being part of Jahiliyyah, the time of pre-Islamic ignorance. Al Ass accordingly ordered that the books be burned to heat the baths for the Muslim warriors.
The dates of the fall of Alexandria are disputed, and Muslims (and Bernard Lewis) deny that Omar destroyed the library, describing it as Christian Crusader propaganda. The account of the destruction of the library comes from mainly Muslim historians, such as Abd Al Latif, Al Qifti, Al Makrizi, Ibn Al Nadim, Georgy Zeidan, and Ibn Khaldun. Lewis argues that, since there are similar accounts of a Persian library that was likewise ordered destroyed by Caliph Omar, both stories are "folklore." And, in an unintentionally anthropic argument, the fact that there are not non-Muslim books that describe the mass burning of non-Muslim books is given as evidence that the burning did not take place.
Previous Days in the "Religion of Peace™":
Sept 16: Black September
Sept 15: Ottomans conquer Persia
Sept 14: Assassination of Gemayel
Sept 13: Confiscation of Armenian property
Sept 12: Salvation of Vienna