"The Roman general Julius Caesar was forced to set fire to his own ships during the Siege of Alexandria in 48 BC. Many ancient writers report that the fire spread and destroyed at least part of the Library of Alexandria's collections; however, the Library seems to have either at least partially survived or been quickly rebuilt.
"In 48 BC, during Caesar's Civil War, Julius Caesar was besieged at Alexandria. His soldiers set fire to his own ships while trying to clear the wharves to block the fleet belonging to Cleopatra's brother Ptolemy XIV. This fire spread to the parts of the city nearest to the docks, causing considerable devastation. The first-century AD Roman playwright and Stoic philosopher Seneca the Younger quotes Livy's Ab Urbe Condita Libri, which was written between 63 and 14 BC, as saying that the fire started by Caesar destroyed 40,000 scrolls from the Library of Alexandria. The Greek Middle PlatonistPlutarch (c. 46–120 AD) writes in his Life of Caesar that, "[W]hen 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." The Roman historian Cassius Dio (c. 155 –c. 235 AD), however, writes: "Many places were set on fire, with the result that, along with other buildings, the dockyards and storehouses of grain and books, said to be great in number and of the finest, were burned." However, Florus and Lucan only mention that the flames burned the fleet itself and some "houses near the sea".
"Scholars have interpreted Cassius Dio's wording to indicate that the fire did not actually destroy the entire Library itself, but rather only a warehouse located near the docks being used by the Library to house scrolls.Whatever devastation Caesar's fire may have caused, the Library was evidently not completely destroyed. The geographer Strabo (c. 63 BC–c. 24 AD) mentions visiting the Mouseion, the larger research institution to which the Library was attached, in around 20 BC, several decades after Caesar's fire, indicating that it either survived the fire or was rebuilt soon afterwards. Nonetheless, Strabo's manner of talking about the Mouseion shows that it was nowhere near as prestigious as it had been a few centuries prior. Despite mentioning the Mouseion, Strabo does not mention the Library separately, perhaps indicating that it had been so drastically reduced in stature and significance that Strabo felt it did not warrant separate mention. It is unclear what happened to the Mouseion after Strabo's mention of it."
(My emphasis - more @ the URL)
So - Did J. Caesar actually destroy the entire library? The sources go both ways, but the milder forms indicate that Caesar damaged the library, but did not destroy it completely. It's an interesting question.