Four times???
How did it manage to catch fire four times???
Not everybody agrees but over the years it has been blamed on
1.
Caesar's conquest in 48 BC
The ancient accounts by Plutarch, Aulus Gellius, Ammianus Marcellinus, and Orosius agree that Caesar accidentally burned the library down during his visit to Alexandria in 48 BC.
Plutarch's Parallel Lives, written at the end of the 1st or beginning of the 2nd century AD, describes the Siege of Alexandria in which Caesar was forced to burn his own ships:
when the enemy endeavored to cut off his communication by sea, he was forced to divert that danger by setting fire to his own ships, which, after burning the docks, thence spread on and destroyed the great library.
—Plutarch, Life of Caesar
2. The library seems to have been maintained and continued in existence until its contents were largely lost during the taking of the city by the
Emperor Aurelian (270–275), who was suppressing a revolt by Queen Zenobia of Palmyra (ruled Egypt AD 269–274).[26] During the course of the fighting, the areas of the city in which the main library was located were damaged.[2] The smaller library located at the Serapeum survived, but part of its contents may have been taken to Constantinople to adorn the new capital in the course of the 4th century. However, Ammianus Marcellinus, writing around AD 378 seems to speak of the library in the Serapeum temple as a thing of the past, and he states that many of the Serapeum library's volumes were burnt when Caesar sacked Alexandria. As he says in Book 22.16.12–13:
Besides this there are many lofty temples, and especially one to Serapis, which, although no words can adequately describe it, we may yet say, from its splendid halls supported by pillars, and its beautiful statues and other embellishments, is so superbly decorated, that next to the Capitol, of which the ever-venerable Rome boasts, the whole world has nothing worthier of admiration. In it were libraries of inestimable value; and the concurrent testimony of ancient records affirm that 70,000 volumes, which had been collected by the anxious care of the Ptolemies, were burnt in the Alexandrian war when the city was sacked in the time of Caesar the Dictator.
—Marcellinus, Ammianus (1862), "Roman History: book 22.16.12–13", in Yonge, C.D., Roman History, London: H.G. Bohn
Decree of Theodosius, destruction of the Serapeum in 391
Paganism was made illegal by an edict of the Emperor Theodosius I in 391. The holdings of the Great Library (both at the Mouseion and at the Serapeum) were on the precincts of pagan temples. While this had previously lent them a measure of protection, in the days of the Christian Roman Empire, whatever protection this had previously afforded them had ceased.[2] The temples of Alexandria were closed by Patriarch Theophilus of Alexandria in AD 391.[27]
Socrates of Constantinople provides the following account of the destruction of the temples in Alexandria, in the fifth book of his Historia Ecclesiastica, written around 440:
At the solicitation of Theophilus, Bishop of Alexandria, the emperor issued an order at this time for the demolition of the heathen temples in that city; commanding also that it should be put in execution under the direction of Theophilus. Seizing this opportunity, Theophilus exerted himself to the utmost to expose the pagan mysteries to contempt. And to begin with, he caused the Mithreum to be cleaned out, and exhibited to public view the tokens of its bloody mysteries. Then he destroyed the Serapeum, and the bloody rites of the Mithreum he publicly caricatured; the Serapeum also he showed full of extravagant superstitions, and he had the phalli of Priapus carried through the midst of the forum. [...] Thus this disturbance having been terminated, the governor of Alexandria, and the commander-in-chief of the troops in Egypt, assisted Theophilus in demolishing the heathen temples.
—Socrates; Roberts, Alexander; Donaldson, James (1885), "Socrates: Book V: Chapter 16", in Philip Schaff et al., Nicene and Post-Nicene Fathers, II, II
Arabic sources
In 642, Alexandria was captured by the Muslim army of
Amr ibn al `Aas. There are five Arabic sources, all at least 500 years after the supposed events, which mention the fate of the library.
Abd'l Latif of Baghdad (1162–1231) states that the library of Alexandria was destroyed by Amr, by the order of the Caliph Omar.
The story is also found in Al-Qifti (1172–1248), History of Learned Men, from whom Bar Hebraeus copied the story.
The longest version of the story is in the Syriac Christian author Bar-Hebraeus (1226–1286), also known as Abu'l Faraj. He translated extracts from his history, the Chronicum Syriacum into Arabic, and added extra material from Arab sources. In this Historia Compendiosa Dynastiarum he describes a certain "John Grammaticus" (490–570) asking Amr for the "books in the royal library." Amr writes to Omar for instructions, and Omar replies: "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."
Al-Maqrizi (1364–1442) also mentions the story briefly, while speaking of the Serapeum.
There is also a story in Ibn Khaldun (1332–1406) which tells that Omar made a similar order about Persian books.