On the eve of the Ides of March, Julius Caesar dined with his friend and ally Decimus Brutus, a distant relation of the more famous Marcus Junius Brutus. He was feeling on edge, for the movement of birds and behaviour of horses had lately been ominous. Decimus Brutus, sensing Caesar’s unease, did his best to persuade him that, regardless of the portents, he was obliged to attend the senate the following day. It would be his last chance to speak before he left to invade Parthia.

In the morning, Caesar made his way, as promised, to the Portico of Pompey and began to gather in the latest petitions. It was a little before midday when Tillius Cimber stepped forward and laid hold of his toga, allowing his fellow senator, one of the Casca brothers, to strike the virgin blow. Decimus Brutus’s gladiators had been instructed to remain on hand. They were hardly needed. Thrust – there was Bucolianus. Thrust – there was Cassius. Thrust – Brutus got Caesar in his thigh. Kai su teknon; Et tu, my child?

No one knew precisely how many conspirators were involved in the assassination. The number was difficult to gauge when even innocent men scrambled to claim a part in it. The list of the guilty drawn up in the aftermath of the act featured names that are still famous today and names that have since faded into obscurity. Cassius Parmensis – a 30-year-old sailor, poet and playwright – is certainly one of the latter.

Stothard’s portrait of Parmensis as a tragic poet and keen Epicurean philosopher certainly humanises him

It is doubtful that Parmensis, “one of the lesser wielders of the daggers … one of the common herd of conspirators”, as Sir Peter Stothard describes him, would have made such an intriguing subject for a book had he not been – according to at least one ancient source – the last of the known stabbers to die. I dare say that the life of the nth killer with a name no one can remember would have attracted little interest from a commercial publisher. As it is, Stothard, former editor of The Times and TLS, has stopped short of writing a biography of the fugitive Parmensis in favour of placing him at the very centre of his vivid narrative of the civil wars that followed Caesar’s death.

These wars, fought predominantly in the seas off Greece, were a grim but inevitable consequence of what Stothard rightly calls “a plot hatched in code and philosophical controversies whose instigators agreed on little but the need to kill Caesar and the time and place for the deed to be done”. Drawn together by the vague notion that Rome could revert to the way it had been once the dictator was dead, the conspirators effectively dug their own graves by failing to establish a proper plan for what to do next. The murder was the easy part. It was the chaos that followed that undid them.

As a law, the Lex Pedia, was issued from Rome to track down and punish the guilty, the assassins went their separate ways. Basilus absconded to Picenum on the Adriatic coast. Parmensis entrusted his life to the sea. Trebonius got as far as Smyrna, on the west coast of modern Turkey, before the soldiers caught up with him, removed his head, and kicked it to the base of a statue of Caesar. They had their first assassin.

It was at Smyrna that Brutus and Cassius reconvened at the end of 43 BC. In the new year, they would meet Caesar’s ally, Mark Antony, and Caesar’s adopted son and heir, Octavian, at Philippi in what would be a disastrous battle for them both. Cassius, who had always felt that they ought to have seen off Mark Antony as well, took his own life. Brutus followed. His head was shipped back to Rome for Octavian’s pleasure but sank to the seabed in a storm.

Much of the power of this book derives from the contrast drawn throughout between the tense life of the conspirators in hiding and the worlds they left behind. Parmensis was named for his hometown of Parma where, Stothard writes, “There were nuts and porcupine quills to pick, hot springs in which to bathe”, and farms framed by the “edgeless tracks” of the “shifting River Taurus”.

While Parmensis waited anxiously on a ship to pick up who he could from Philippi, his townspeople were in mourning, raped and flogged by the avenging Romans. The people of Parma had once defended themselves from the Gauls with the little parmulae shields from which they allegedly took their name. Shields were of little use to them now.

Stothard is also good on the shifting allegiances of the men who jostled for power while the hunt for the assassins continued. It was in Parmensis’s interest, he explains, that Sextus Pompeius, the surviving son of Caesar’s old foe Pompey the Great, should forge a close tie with Mark Antony, for then Octavian would be more likely to falter. The longer Parmensis could remain aloof, the more distractions arose from would-be tyrants, the greater the likelihood that he would eventually be forgotten.

For a fleeting moment, the reader thinks – even hopes – that Parmensis might get away with it. A mock trial of his co-conspirators Brutus and Cassius hosted by the charity Classics for All at the British Supreme Court a few years ago resulted in their surprise acquittal by the audience jury. The assassins have somehow in death succeeded in attracting us to their cause against the power-grabbing, warmongering, womanising but ultimately tragic Julius Caesar.

Stothard’s portrait of Parmensis as a tragic poet – a Thyestes was among his works – and keen Epicurean philosopher certainly humanises him. He emerges, indeed, as a most sensitive killer. If his initial motivations remain shadowy, then his gumption and determination to stay alive – or at the very least stare death honourably in the face – reverberate clearly through Stothard’s tense and thrilling narrative.