This brief piece from the History Channel (2013) blames Julius Caesar for the destruction of the Library of Alexandria as if this is a proven fact.
Whatever the case, it's true that Caesar was widely blamed at the time:
"The prime suspect in destruction of the Library of Alexandria is Julius Caesar. It is alleged that during Caesar's occupation of the city of Alexandria in 48 BCE, he found himself in the Royal Palace, hemmed in by the Egyptian fleet in the harbour. For his own safety he had his men set fire to the Egyptian ships, but the fire got out of control and spread to the parts of the city nearest the shore, which included warehouses, depots and some arsenals." Source: Ancient History Encyclopedia.
That's the story, anyway, but it probably isn't true:
"In his book The Vanished Library, Luciano Canfora interprets the evidence from ancient writers to indicate the destruction of manuscripts stored in warehouses near the port waiting for export, rather than the great Library itself." Furthermore, "[t]he great scholar and stoic philosopher Strabo, was working in Alexandria in 20 BC and from his writings it is obvious that the Library was not at that time the world-renowned centre for learning it had been in previous centuries." (Ancient History Encyclopedia)
Clip from Cosmos (1980). Note the standard assumption that the library was deliberately destroyed.
The Christians have been blamed as well, but it doesn't look like they did it, either:
"The second possible culprit would be the Christians of the 4 th century AD. In 391 AD, the Emperor Theodosius issued a decree that officially outlawed pagan practices. Thus, the Serapeum or Temple of Serapis in Alexandria was destroyed. However, this was not the Library of Alexandria, or for that matter, a library of any sort. Furthermore, no ancient sources mention the destruction of any library at this time at all. Hence, there is no evidence that the Christians of the 4 th century destroyed the Library of Alexandria." Source: Ancient Origins.
So if Caesar wasn't responsible and the Christians seem to have been exonerated, who destroyed the library and its priceless collection of ancient texts? Strangely enough, considering how widespread the traditional story is, there's a possibility that the library wasn't destroyed by a fire in the first place. The truth may be radically different and a lot less dramatic.
"Though it seems fitting that the destruction of so mythic an institution as the Great Library of Alexandria must have required some cataclysmic event ... – and while [riots, fires and other disasters] certainly took their toll on the Library - in reality, the fortunes of the Great Library waxed and waned with those of Alexandria itself. Much of [the library's] downfall was gradual, often bureaucratic, and by comparison to our cultural imaginings, somewhat petty." Source: "The Great Library Of Alexandria?" by Heather Philips, Library Philosophy and Practice, University of Nebraska-Lincoln Libraries.
This article appeared in 2010, I think. If Heather Philips is right, her version of events would radically change the traditional history of the destruction of the Great Library, but that would be a good thing for those of us who want to know the true story. The traditional history has always been highly speculative since no one really knows what happened.
If Philips is right, we'd lose some dramatic stories, though. For example, her revisionist view would demolish the historical accuracy (such as it is) of the excellent 2009 movie, Agora, which tells the heavily romanticized story of Hypatia of Alexandria and blames the destruction of the library on a rioting Christian mob.
Agora is a good flick, highly recommended, but it's undeniably a work of anti-Christian propaganda. That doesn't bother me particularly, but some people get all worked up about it. For instance, here's a harsh attack on the movie (2010) by David Bentley Hart who admits he never saw the film but argues that its premise is completely ahistorical, which may very well be true. Considering their record of destroying pagan shrines and writings, I wouldn't be surprised to learn that the Christians destroyed the library, but it looks like they weren't involved.
Various other individuals and groups have been blamed for the destruction of the Great Library --the emperor Aurelian, the Muslims, Pope Theophilus of Alexandria -- but if Heather Philips is right, the library may never have been "destroyed" at all, only damaged over the years in various wars and riots before it went out of business due to budget cuts. That doesn't make for a good movie, however, so if that's what really happened, I doubt if the story will ever make it to the Big Screen.