If there’s a bright spot in the murky mess of Iraq, it’s that finally we have a war that it is impossible to paint in simple terms, as a battle of good against evil. This time, even our PM, the self-appointed heir to Blair, can’t grandstand about defeating ‘terror’ or protecting ‘innocent civilians’ because there’s terror and innocence on every side. He can’t pose as world policeman; stand side by side with Obama and say ‘we must not let this evil happen’, because clearly we already have.

Take ISIS, the Islamist group once affiliated to al-Qa’eda who’ve become the world’s new public enemy number one. ISIS have captured parts of northeast Syria and Iraq, and have begun to eye up Baghdad. They’re into beheading and stoning, even crucifying civilians, and under other circumstances it’d be tempting for both Obama and Cameron to paint them as the very apex of evil and the antithesis of all the West holds dear.

But as both leaders know, ISIS are not the only horror show around. Take a look at the ‘good’ rebel soldiers of the Free Syrian Army, the ones we’ve armed and backed against Assad, and you’ll see they’ve developed quite a taste for darkness too. You can watch them on YouTube, if you like, sawing off body parts from their Shia victims and wearing necklaces made of ears. The brutality has spread as if waterborne down the Euphrates.

Next, consider all the ways in which ISIS owes its success to the West. They’re a tiny outfit, just a few thousand men, but they’ve been able to capture great swaths of Iraq with remarkable ease, because ordinary Iraqis don’t care enough to fight. This is in part our fault. The man we backed, Prime Minister Nouri al-Maliki, has presided over the collapse and corruption of government and the police. He’s sidelined and bullied Iraq’s Sunnis to the extent that they now don’t much care who’s in charge. The eccentric decision after the Iraq war to exclude Ba’ath Party members from the new government has meant that Ba’athists in particular have been only too happy to help ISIS on their way.

It’s a dark and twisted business, this new battle for Baghdad. This isn’t to say we shouldn’t act to save the city, but just that any talk of goodies and baddies makes little sense. Here’s how surreal the situation has become: some of the Shia who normally fight in Syria for Assad (against the US-backed FSA) have trooped south to do battle with ISIS in Iraq (on America’s side). As they crossed the border, I suppose, they changed from foe to friend, and they’ll change back again when, if, they return. Cameron’s been telling us for years about the need to depose the tyrant Assad. Now we rely on the tyrant’s troops.

Our other allies are if anything even less reassuring. For some time now Mr Maliki has been under the sheltering wing of Iran. After ISIS took Mosul last week, several Iranian units were deployed to defend the capital and protect the Shia shrines in Samarra to the north. Though Obama may send in his drones, the man really leading the charge against ISIS will be the impressively ruthless-looking Major General Qassem Suleimani, head of the Iranian Revolutionary Guards’ elite Quds Force, who arrived in Baghdad last week to direct operations.

Anyone still tempted to make clear moral distinctions might recall Hillary Clinton’s comments about Iran when she ran for her party’s presidential nomination in 2008. ‘If I’m the president, we will attack Iran’ if it attacks Israel. Then, with a cackle, she spoke of ‘obliterating’ it. Won’t it be curious, come 2016, if Hillary’s presidential dreams come true and she finds herself with Iran as an ally, still fighting shoulder to shoulder in Iraq?

Here’s how interchangeable the words ‘friend’ and ‘enemy’ have become. As America prepares to help Iran beat back ISIS, so also this week it meets with Iranian ministers to give them a talking-to about their nuclear programme; to wave more sanctions in their face. It’s like hobbling a horse you’ve backed to win. Meanwhile: America still hands Saudi Arabia billions of dollars a year — and they hand it on to ISIS to buy guns.

I say it’s impossible to paint a black and white picture, but I’m aware there’s another form of Manichaeism waiting in the wings. In the absence of other certainties, it’s become common to say: look what happens when men are driven mad by a medieval religion. Look what animals the Islamists have become without reason to light their way. There’s some truth there — but again no easy them and us. A few thousand miles to the west, Christians hack Muslim children to death with machetes in the Central African Republic. In Mexico, atheistic drug cartels leave the beaches strewn with severed heads. The US colonel Kenneth King, who ran a camp in Iraq for both Sunni and Shia inmates, described the feuding between them in gang terms: ‘It’s the Bloods with the Bloods and the Crips with the Crips, that kind of thing.’

We often paint our enemies as deranged, especially by Islam, as if it’s comforting to see ourselves as uniquely sane. So before we write off ISIS as madmen, let’s be clear: they saw a chance in the chaos we helped create and they seized it. Now they control the beginnings of a caliphate from northeast Syria down into Iraq. What’s so crazy about that?

Even the ultra-violence has its logic. It’s evil, yes, but not insane. ISIS relies on its reputation. Such a tiny brigade could never have made such headway otherwise. They’ve taken towns in Iraq, from Mosul to Baiji, by simply calling ahead and saying: you know what we’re like, so flee or face the consequences.

Just in case, in coming weeks, Cameron’s still tempted to pose as saviour, let’s remember that this new Iraq war plays out against the backdrop of his last one, ongoing in Libya. In we flew in 2011, short on planning, high on talk of protecting innocents. Liberated Libya is now a lawless drug-infested hell; a playground for ISIS and its like. The free men of Benghazi quite openly now rue the day we arrived to ‘save’ them.