Overview
Rome burned regularly. It was a city of perhaps a million people living in timber-framed tenements packed along streets narrow enough to touch across, with open cooking fires on every floor and no meaningful firebreaks. What began near the Circus Maximus on the night of 18 July 64 AD was not unusual in kind, only in scale: six days of burning, a brief containment, then a further outbreak. Ten of the fourteen districts were damaged and three were left with nothing standing.
The emperor was thirty miles away at Antium. He returned to the city, opened public buildings and his own gardens to the homeless, brought in food and cut the price of grain. He also, afterwards, built himself a very large palace on ground the fire had cleared.
The Rumour
The accusation was in circulation almost immediately, and Tacitus preserves it: that the fire had been set on the emperor's orders, and that while it burned he had appeared on a private stage and sung of the destruction of Troy.
It is worth being exact about what the sources are. Tacitus was about eight years old in 64 and wrote the Annals half a century later; he reports the story as rumour and does not endorse it. Suetonius, writing later still and with a frank appetite for imperial scandal, states it as fact and has Nero singing in costume. Cassius Dio, later again, does the same. The fiddle is a much later embellishment — the instrument did not exist — and the lyre or cithara is the version the ancient sources actually describe.
The motive attributed was the Domus Aurea, the Golden House: an enormous palace complex with an artificial lake and a colossal statue, built across ground in the centre of Rome that the fire had conveniently emptied. To Romans watching an emperor take possession of the burnt district, the sequence spoke for itself.
The Case Against It
Modern historians have generally treated Nero's responsibility as unlikely, on grounds that do not require any sympathy for him.
The fire began in an area of shops around the Circus Maximus, not in the district he would later build on, and it destroyed a great deal that he had reason to value, including parts of his own existing palace and irreplaceable temples. He was out of the city when it started, which is a poor arrangement for an arsonist and an equally poor alibi in the eyes of a suspicious population. And his conduct during the emergency was, by Tacitus's own account, energetic and effective.
Set against that is the simplest explanation available: a wooden city in July, a fire near a district full of oil and cloth, and no fire service capable of stopping it once the wind took hold. Rome had burned before and would burn again.
The Christians
What happened next is the part with the longest consequences. Tacitus writes that to dispel the rumour, Nero fastened the guilt on a class hated for their abominations, called Christians by the populace, and that they were convicted less for the crime of arson than for hatred of the human race. They were torn by dogs, crucified, and burned to serve as lighting after dark.
The passage in Annals 15.44 is among the most consequential in ancient literature. It is the earliest surviving non-Christian reference to the execution of Christ under Pontius Pilate, and it establishes a Christian community in Rome large and distinct enough to be blamed for a disaster. It also supplies the founding episode of Christian persecution narrative, and by tradition the context in which Peter and Paul died.
The scapegoating is the element of the affair least disputed by historians, and it is a documented conspiracy in the ordinary sense: a deliberate attribution of a crime to a group known not to have committed it, in order to deflect suspicion from the person under it.
Nero Redivivus
The emperor killed himself in 68 AD, at thirty. A substantial part of the empire's eastern population declined to accept it. The belief that he had escaped and would return at the head of a Parthian army persisted for decades and produced at least three impostors who attracted followings.
The expectation left traces in Christian apocalyptic writing, where a returning Nero is widely read behind the figure of the beast in Revelation whose mortal wound was healed. The theory that an emperor faked his death and is waiting to come back is not a modern invention, and its first well-documented instance concerns a man who really had died, in front of witnesses, four years after the city burned.