In AD 42 the incumbent ruler of the Roman Empire, best known to posterity as Caligula, was assassinated in a conspiracy formed by senators and members of his nominal bodyguard, the Praetorian Guard. He was Rome’s third emperor and the first to be removed by the unit sworn to protect him. In the bloodletting that followed his death, the sanctity of the imperial family was disregarded and his wife and daughter were murdered. But the Praetorian Guard had a wider role to play than simply that of king-slayers: guardsmen found Caligula’s uncle, Claudius, in the palace (cowering behind a curtain, if Suetonius is to be believed) and had him declared emperor by the Senate. Their power to make – or break – emperors was on public display. ‘Such formidable servants’, Edward Gibbon remarked in The Decline and Fall of the Roman Empire, ‘are always necessary, but often fatal to the throne of despotism.’

Those ‘formidable servants’ – and how fatal they often proved to be to the fortunes of emperors – are the focus of Guy de la Bédoyère’s Praetorian. He provides a potted history of the Roman Empire, told through the triumphs and travails of the praetorians. In the late days of