Where Tim discusses fourth century Roman history. Note, at this time, the Empire was well used to having more than one Emperor.

The Emperor Constantius II was a right bastard. The massacre of the princes - where he killed off his male relatives in Constantinople during a family gathering following the death of his father Constantine The Great in 337 - was just the sort of ‘real’ history that gives Game of Thrones legitimacy.

One nephew that survived the cull was Julian. A bookish and pious prince, he was spared because he was so young and, well, a bit of a nerd. But ten years later - following the overthrow of Western emperor Constans – cousin Constantius needed a partner to share in the burden of the imperial purple. Turning first to Gallus, Julian's older brother – who he later killed - Constantius eventually elevated Julian into the family business as Caesar of the West in 355.

Here the boy became a man. After kicking some serious German butt, Julian became popular with his legions. Cousin Constantius got jealous and there followed lots of 'come and have a go if you think you're hard enough' correspondence between the two emperors until Julian marched East at the head of an army in 361. And then – miraculously - Cousin Constantius died leaving young Julian the sole master of the Roman world. What to do?

Well, what Julian did - in his brief two year reign – was turn the clock back on Christianity and attempt to re-establish the old gods. You know, get rid of all this Christian rubbish legitimised by Constantine. He also thought Persia was up for a bit of Roman steel and so marched off deep into the Sasanian Empire, never to return. Killed by a random spear, Julian left his troops miles from safety on the Euphrates and in the feeble hands of his short-lived successor Jovian.

So why do I tell the story of Julian the Apostate?

Well, unlike his uncle Constantine (the Great), he only had 2 years to make his mark. Constantine had 31 – with 13 in sole charge of the Empire. Constantine changed the course of history. Julian flamed out quickly and his successors Valentinian, Valens and Theodosius reaffirmed the Christian hegemony (give or take the odd Arian, or semi Arian, heresy). Julian was an anomaly and Western history writes that Constantine looms large whereas Julian does not.

Can one person change the course of history? Or – as in this case – a solitary spear? What if Julian had lived and reigned twenty years? Would he have quashed Christianity and reduced it into a cult, one of many, like Isis, Mithra or Sol Invictus, that bubbled around in the later Roman Empire? It’s possible that Christianity could have gone underground only to re-emerge stronger, much as it did during the persecution of Diocletian sixty years earlier. It’s impossible to say. It’s a little like powerful newspapers; do they lead opinion or merely reflect it?

What’s of interest though for those who seek parallels in history, who look for patterns to help with understanding the present day, is the theory that there are turning points – yes kings and emperors – but social, religious, military too, that alter the course of history. The trick is to spot whether events have produced a Constantine the Great or a Julian The Apostate.