THE sickening truth about the Islamic State’s bloodthirsty reign of terror has been uncovered in the paper trail left behind in Iraq’s ravaged city.

A fascinating trove of documents found by the New York Times suggest the jihadists flourished not because of oil, as was long believed, but thanks to their twisted version of ordinary government.

The terrorists controlled citizens through a program of punitive taxes, fines for so-called immorality and taking land and property from families to give to its fighters.

And the files collected from the rubble of bombed buildings after IS was flattened by the Iraqi government suggest a horrifying possibility: next time may be worse.

One of the documents revealed a suicide bomber having to fill out a form saying they would payback a Moroccan family $600 before acting.

The terror group believes jihadists cannot enter paradise until they have no “worldly debts”.

VIOLENT ENDS

Like the Nazis before them, IS operated with brutal, military precision. A teenage girl accused of adultery had a stone slab dropped on her head in the street. A woman who didn’t cover her face was punched in the eye. Suspected spies were hanged in public spaces.

People were imprisoned for misdemeanours including eyebrow plucking, the wrong haircut, playing cards, listening to music and smoking hookah.

Government workers were so afraid they could be thrown in jail or executed for the smallest misdemeanour that Iraq operated more smoothly than it ever had before — in total fear.

The documents show the terrorists ran a complex system, overseeing fertility examinations for couples, issuing birth certificates, granting drivers’ licences and even printing its own stationery featuring a black flag.

They also learned from the mistakes of the past. Rather than purging institutions, thus creating a power vacuum — as the US did after Saddam Hussein’s downfall — the terrorist organisation forced existing structures, making civilians and state troops do its bidding in Iraq and Syria.

Despite the terrorists’ taste for ghastly televised beheadings, they were not simply savages, but more organised than many administrations today. That’s why Donald Trump’s military advisers have warned that the US withdrawal from Syria he is pushing for could result in an Islamic State resurgence.

GOVERNMENT OF DEATH

The extraordinary IS files gathered by The New York Times show meticulous records of taxes on agriculture, which earned them millions of dollars alone. The militants were particularly hard to defeat because of their diverse revenue streams, meaning cutting off access to oil by bombing plants failed miserably to stop them.

In the end, this was a bitter battle that cost the West dearly, finally ending last year after thousands of Coalition deaths in the Middle East.

We now seem to be in a lull, but the war is not over. Experts warn that IS has created a blueprint that will be used again.

Osama bin Laden’s caliphate dream was based on ancient Islamic society, with a dystopian twist. Now, government departments included the morality police, an office for pillaging antiquities and one for dividing up the spoils of war. Beautiful historic artefacts were destroyed and plundered, never to be recovered.

This was a grim era for women, who lost their jobs, their identities and their agency — or faced death. Men were instructed to stop shaving and wear religious garb. Everyone was obedient — and afraid.

MINISTRY OF WAR SPOILS

Disturbingly, the documents are evidence that many of those who furthered the jihadists’ deadly goals most effectively were ordinary, frightened workers rather than trained assassins.

A 27-page handbook entitled “The Caliphate on the Path of Prophecy” outlined plans to steal property from banned religions and use them to build the empire. Shia Muslims (IS is Sunni) and others faced the theft of everything they owned.

But Iraqi officers told researchers that these citizens would not be penalised. Iraq’s state troops are still facing a daunting task of weeding out the killers who committed terrible atrocities.

No doubt they will miss some — and a new generation of terrorists may be quietly radicalised behind closed doors.

Perhaps the queasiest reality that the Middle East is now facing is how much better it ran under the authoritative terror group. The streets were cleaner, a new Mosul road named Caliphate Way reduced congestion, power and water were less likely to be cut off, sewers didn’t overflow and road repairs happened fast.

The price to pay was daily bombings and the trauma of death that will haunt generations to come.

Still, IS thrived as the richest terror group of all time, and that requires attention — or we may not be as lucky next time.