In a voice pleading and in despair, the woman who had fled for her life asked: “What century are we in?” She was an Iraqi Christian, reached by the BBC World Service even as she sought to escape the self-declared Islamic State, or IS (formerly Isis). “They will sell us,” she said. “They will rape us.” Her words echoed this week’s tearful warning to the Iraqi parliament from a Kurdish MP who described the fate befalling her fellow Yazidis. “Mr Speaker, our women are being taken as slaves and sold in the slave market.”

The year is 2014 and yet 40,000 followers of a 1,000-year-old faith are huddling on a mountainside said to be the final resting place of Noah’s Ark, fearing their women are to be dragged to a slave market. As the woman asked, what century are we in?

When news reports speak of ancient sectarian loathings, when the gap between Sunni and Shia comes down to a theological dispute originating in the seventh century, when the Islamic State declares its defining mission to be the restoration of a caliphate from the same period, then it is tempting to believe this is indeed the curious fate of our supposedly modern era – that we are being drawn back to a medieval or pre-medieval world of holy war and wholesale slaughter in the name of religion. The irony of it seems so rich: that just as technology is accelerating, making once impossible feats of connection routine, so the clock is turning backward, towards a new dark age of beheadings and enslavement, a fearsome army threatening a tiny sect with that ancient ultimatum – bow to our god or die.

From the vantage point of avowedly secular Britain, where even the most watery form of Christianity has become a minority interest, the persistence of religion is indeed one of the 21st century’s great surprises. For so long, progress and the decline of faith – what enlightened types prefer to call “superstition” – were thought to be symbiotic if not synonymous. As the world advanced, as more of its people got running water, TV and smartphones, surely the old, primitive beliefs would fade. But the Middle East has confounded that now quaint conviction. Large swaths of states that were once secular – the Ba’athist republics of Iraq and Syria used to revere nationalism over Islam – are now under the black flag of IS, ruled by a Qur’anic scholar who has anointed himself caliph.

Yet neat though it is to see return to holy war as the motif of our age, it might be wrong. The rolling advances of IS – brutal and laden with treasure, conquering one city or stronghold after another – may indeed resemble the world of several centuries ago but not in the way we’ve imagined. It is instead a story that is both ancient and very modern.

According to Toby Dodge, the scholar of Iraq at the International Institute for Strategic Studies (IISS), what’s driving IS, or at least making its phenomenal success possible, is not pre-modern religious zeal so much as a pre-modern absence of state power. The state structures of both Iraq and Syria have all but collapsed. The result is a power vacuum of a kind that would have been recognised in the lawless Europe of seven or eight centuries ago – and which IS has exploited with the ruthless discipline of those long ago baronial warlords who turned themselves into European princes.

“Islamic State are jihadis with MBAs,” says Dodge, speaking of a movement so modern it has its own gift shop. He notes its combination of fierce religious ideology, financial acumen and tactical nous. “It’s Darwinian,” he adds, describing IS leader Abu Bakr al-Baghdadi and his inner circle as those strong enough to have survived the US hammering of al-Qaida in Iraq between 2007 and 2009. But what has been crucial, Dodge says, is “not ancient hatreds but this collapse of state power”.

Which partly explains IS’s choice of targets. It attacks wherever it sees a gap, an area of weakness where the state’s writ does not run or that will be too feeble to resist. So when IS’s advance south to Baghdad was repelled, the organisation turned and looked for vacuums to fill. Christian areas were one such target; the remote Sinjar stronghold of the Yazidis is another. With a merciless appetite for territory, IS hunts down any patches of Iraq or Syria it believes can be conquered easily.

In Syria, the degradation of the state has been the consequence of a civil war in which the government of Bashar al-Assad has turned its fire on its own people. In Iraq, the explanation comes in two parts. First, the US-led invasion of 2003 smashed the Saddam state. Second, the current prime minister, Nouri al-Maliki, has hollowed out what was left, eviscerating national institutions lest they pose a challenge to him and his narrowly Shia ruling circle. Most important, he gutted the Iraqi army, seeing a body of one million men under arms as a personal threat rather than a national asset. Little wonder that Mosul, defended by al-Maliki cronies rather than able commanders, fell at the first sight of Isis. Despite its expensive US training, the Iraqi army simply melted away.

The void in Iraq can, then, be doubly blamed on the US. The 2003 invasion is, of course, the original sin. But the manner of the withdrawal in 2011 – gifting state-of-the-art US military hardware worth billions to an army headed by al-Maliki, only for that hardware to fall into the hands of Isis – was clearly a catastrophic error too. The result is that Barack Obama, whose presidency was predicated on a promise to end the war in Iraq, has been drawn into combat once more. His air strikes on IS forces in northern Iraq on Friday make him the fourth US president in succession to order military action in that country. Ronald Reagan was the last one not to drop bombs on Iraq.

Which brings us to the new aspect of the geopolitical landscape. It relates again to the absence of power, this time at the global level. The analyst Ian Bremmer says we live in a G-Zero world, one in which we don’t have one true superpower, let alone two. The US is weaker than at any time since 1945, unable to force a breakthrough in Ukraine or Syria or, most recently, Gaza.

Islamic State may wrap itself in the flag of jihad, but its success owes more to medieval lawlessness than medieval religious enmity – helped by the very 21st-century decline of the global behemoth. Our world is being shaken, but the persistence of religion is more a symptom than a cause. The larger problem, as old as mankind, is power and the lack of it. For sometimes weakness can be just as dangerous as strength.

Twitter: @freedland