A quiet night in the Eternal City. As on most nights - especially winter nights - most people are at home, asleep or huddled beside charcoal braziers. Here and there, bands of vigiles, their legs wrapped in cloth to ward off the chill, file down darkened streets. The rumble of the delivery wagons echoes in the warehouse district by the Tiber. And in the Praetorian camp, mutiny is afoot...

But would you, a humble praetorian, know what would happen the following day? Would you be aware that your superiors were plotting to kill Gaius Caligula, the most powerful man on earth?

Probably not, unless you were important enough to have a direct hand in the plot.

The most detailed account of Caligula's assassination is found in Josephus' Antiquities of the Jews (you can read a full, if rather archaic, translation here). According to Josephus, there were actually several plots - more or less loosely coordinated - to kill Caligula. The plot that succeeded was spearheaded by Cassius Chaerea, a tribune of the Praetorian Guard who hated the emperor for personal reasons. (Caligula mocked Chaerea's squeaky voice, calling him "milady" and making him repeat salacious watchwords).

Chaerea appears to have been encouraged by other interested parties - centered on various senators and imperial freedmen (and perhaps even the future emperor Claudius) - to take vengeance and rid Rome of the tyrant. In the weeks leading up to the assassination, he made at least two of his fellow Praetorian tribunes - Cornelius Sabinus and a man named Papinius - privy to the plot.

But how many among the rank and file knew? According to Josephus, knowledge of the plot was fairly widespread, being known to "senators, equestrians, and those soldiers [presumably the Praetorians] to whom it had been revealed" (19.62). That aggravatingly inexact phrase about the soldiers who knew about the plot (ὁπόσοι τοῦ στρατιωτικοῦ συνῄδεσαν, in case you're achingly curious about the Greek) is our only indication that Chaerea and his fellow tribunes had told at least some of the rank-and-file.

The number of guards involved in the actual assassination, of course, was fairly small. Suetonius suggests, however, that Chaerea and the other conspirators had informed several centurions (58.2); and the fact that Caligula was stabbed 30 times suggests a substantial crowd of assassins (or at least a few very zealous ones). But not everyone around Caligula was aware of the plot: the emperor's litter-bearers and German bodyguards immediately attacked the assassins.

Perhaps the best indication that a substantial number of Praetorians had advance warning of the assassination is the fact that Caligula's wife and daughter were killed before they had a chance to escape the palace. At least some of the units on duty were clearly aware of the plot, and had orders to prevent the imperial family from fleeing.

In the end, we are reduced to common sense. Chaerea and friends probably told everyone who had to know about the plot - and only those who had to know - shortly before the projected time of the assassination. It would, after all, take only a single loyal or incautious soldier to sabotage the whole enterprise.

Even a tyrannical emperor always had supporters - people who depended on, needed, or genuinely liked him. To kill an emperor, conspirators had to win considerable support among those with immediate access to him. But it was never wise to promulgate too widely. The Pisonian conspiracy against Nero (which involved several Praetorian tribunes) failed after an overzealous conspirator told the wrong person. Only if the entire Praetorian Guard rebelled against the emperor - when they revolted against Pertinax, for example, the guard rushed en masse from their camp - would the rank and file be told. Most plots, including the one that ended the reign of Caligula, were more subtle.

On Caligula, see Anthony Barrett's sober Caligula: The Corruption of Power and Balsdon's old but still very readable The Emperor Gaius.