At its peak in 2015, the Islamic State governed between 8 and 12 million people across territory larger than the UK. This is how it happened, what life inside actually looked like, and why the threat didn't end when the caliphate did.
At its peak in 2015, the Islamic State governed between 8 and 12 million people across territory larger than the UK. This is how it happened, what life inside actually looked like, and why the threat didn’t end when the caliphate did.
In the summer of 2014, a man called Abu Bakr al-Baghdadi stood at the pulpit of the Great Mosque of al-Nuri in Mosul and declared the establishment of a caliphate. It was, on the face of it, an absurd claim — a militia leader announcing the restoration of an Islamic empire that had been absent from the world for nearly a century. Most analysts gave it months.
What followed was one of the most remarkable and disturbing experiments in governance of the twenty-first century. Within a year, the Islamic State controlled territory larger than the United Kingdom. It had its own currency, tax system, courts, schools, hospitals, and media production apparatus. It was collecting oil revenues. It was governing, in a brutal and highly organised way, a population estimated at between eight and twelve million people.
This is the story of how that happened, who those people were, and what life inside the Islamic State actually looked like — based on accounts from survivors, journalists who covered the conflict, and the extensive documentation left behind when the caliphate collapsed.
“At its peak, the Islamic State governed more people than many recognised European nations. It was not a terrorist group that held territory. It was a state that also committed terrorism.”
How Many People Lived in the Islamic State?
Precise figures are genuinely difficult to establish, and estimates have varied significantly depending on the methodology and the moment in time. The most widely cited figure is that by mid-2015, at the peak of ISIS territorial control, approximately eight to twelve million people lived within the borders of the self-declared caliphate.
The UN and various research institutions have used different approaches — satellite imagery, population density data, refugee interviews, and on-the-ground reporting. The IHS Conflict Monitor, which tracked ISIS territorial control in detail, estimated the population under ISIS control at its peak at around 8 million. Other estimates, accounting for more loosely controlled territory, pushed that figure higher.
The territory itself was vast. At its maximum extent in mid-2014 to 2015, the Islamic State controlled an area of approximately 88,000 square kilometres — stretching from the outskirts of Aleppo in western Syria to the towns of Diyala province in eastern Iraq. It included Mosul, Iraq’s second-largest city, with a pre-war population of nearly two million people. It included Raqqa, which became the de facto capital. It included Fallujah, Tikrit, and large sections of the Euphrates and Tigris river valleys.
The army that controlled this territory numbered somewhere between 20,000 and 31,500 fighters at peak strength, according to CIA estimates — a remarkably small force relative to the population it governed, which says something about how ISIS maintained control: not primarily through physical force, but through a combination of ideological enforcement, administrative co-optation, and the calculated use of extreme violence as a deterrent.
How Did ISIS Build a State So Quickly?
The Islamic State did not emerge from nothing. Its roots lay in Al-Qaeda in Iraq, founded by Abu Musab al-Zarqawi in the chaos that followed the 2003 US invasion. That organisation was brutally suppressed during the US surge of 2007-2008, but it survived underground — and the conditions that had produced it were never resolved.
When the Syrian civil war began in 2011, it created the vacuum that ISIS needed. The Assad government’s deliberate strategy of releasing jihadist prisoners while imprisoning secular opposition activists seeded the Syrian insurgency with experienced extremists. The collapse of Iraqi state authority in Sunni-majority areas — where the Maliki government’s sectarian policies had alienated the population — provided the social base. The capture of enormous quantities of US-supplied military equipment from fleeing Iraqi army units in 2014 provided the military means.
The fall of Mosul in June 2014 was the turning point. An estimated 30,000 Iraqi army soldiers fled before a force of roughly 1,500 ISIS fighters. The Iraqi army’s collapse was not primarily military — it was the result of years of politicisation, corruption, and the hollowing out of non-sectarian institutions. ISIS captured weapons, vehicles, cash from the central bank, and the psychological momentum that came from defeating a US-trained army in a matter of days.
What distinguished ISIS from other jihadist groups was what it did next. It didn’t just hold territory. It administered it.
Life Inside the Islamic State
The accounts of people who lived under ISIS rule — civilians who could not flee, families with nowhere to go, people caught on the wrong side of a rapidly moving frontline — describe something that defies simple characterisation.
ISIS imposed an extraordinarily strict interpretation of Islamic law. Music was banned. Smoking was banned. Women were required to wear full face coverings in public and could not leave home without a male guardian. Men were required to grow beards of a specified length. Churches were destroyed or converted. The Yazidi religious minority faced what the UN has since characterised as genocide — mass executions of men, systematic sexual enslavement of women and girls.
And yet, for ordinary Sunni Arab civilians in areas that had already been destabilised by years of sectarian violence, ISIS also brought a kind of order. Checkpoints worked. Courts settled disputes. Prices were regulated. Electricity and water were maintained in some areas. Schools — teaching an ISIS-approved curriculum — remained open. This is not an apology for ISIS. It is an explanation of how twelve million people came to live under its rule without universal resistance: because the alternative, for many of them, had already been demonstrated to be worse.
“ISIS brought a brutal order to places that had known only chaos. For civilians who had lived through years of sectarian violence, that calculation was not simple.”
The Foreign Fighters
One of the most discussed aspects of the Islamic State was the foreign fighter phenomenon. An estimated 40,000 people from over 110 countries travelled to Syria and Iraq to join ISIS between 2013 and 2016 — the largest mobilisation of foreign fighters to a single conflict since the Soviet-Afghan war of the 1980s.
They came from France, Belgium, the UK, Germany, Australia, the United States, Russia, Tunisia, Saudi Arabia, and dozens of other countries. Some were ideologically motivated — drawn by the caliphate narrative, by ISIS’s sophisticated propaganda, by a sense of purpose and belonging that they hadn’t found elsewhere. Some came for adventure. Some came because they had been radicalised online and had no meaningful social connection to their home countries. Some brought their families.
The foreign fighter wave had significant intelligence implications. When ISIS began to collapse, thousands of experienced combatants dispersed across the globe — returning home, moving to other conflict zones, going underground. Tracking them became one of the primary security concerns of intelligence agencies in Europe and North America for years afterward.
The Collapse of the Caliphate
The territorial caliphate lasted approximately five years from its formal declaration in June 2014 to the fall of Baghouz in March 2019, when Syrian Democratic Forces — primarily Kurdish fighters backed by a US-led coalition — cleared the last ISIS-held territory in Syria.
The military campaign against ISIS was long, costly, and ultimately successful in its primary objective of destroying the territorial state. Mosul was retaken by Iraqi forces in July 2017 after nine months of some of the most intense urban combat since the Second World War. The battle for the old city alone — street by street, building by building — resulted in massive civilian casualties and left large sections of one of Iraq’s most historically significant cities in ruins. Raqqa was retaken by the SDF in October 2017 after a similar siege.
By early 2019, the caliphate had lost 99% of its territory and the vast majority of its population. Al-Baghdadi himself died in October 2019 during a US special forces raid in northwestern Syria, detonating a suicide vest as forces closed in on his hiding place.
Is ISIS Still a Threat?
The territorial caliphate is gone. The Islamic State is not. This is an important distinction that security analysts have been making consistently since 2019, and which events since then have repeatedly validated.
ISIS operates today as a clandestine insurgent organisation in Iraq and Syria, conducting guerrilla attacks, assassinations, and periodic mass-casualty bombings. In Iraq’s Diyala and Kirkuk provinces, it has maintained a persistent low-level insurgency that has proven resistant to counterterrorism operations. In Syria’s Deir ez-Zor region, it retains networks that the SDF and its US partners have been unable to fully suppress.
Beyond Iraq and Syria, ISIS has established or inspired affiliate organisations in approximately 19 countries across Africa and Asia. ISIS-Khorasan (ISIS-K), the Afghan affiliate, has been responsible for several high-profile attacks including the bombing at Kabul airport during the 2021 US withdrawal that killed 183 people, and the attack on the Crocus City Hall concert venue in Moscow in March 2024 that killed 145 people — one of the deadliest terrorist attacks in Europe in two decades.
ISIS in Africa — particularly through its Sahel, West Africa, and Mozambique branches — has emerged as a growing threat as the organisation has lost ground in its original heartland and shifted resources and personnel toward the continent. The Sahel in particular has seen a dramatic deterioration in security as ISIS and al-Qaeda affiliated groups have expanded their territorial control in Mali, Burkina Faso, and Niger.
What the Caliphate Left Behind
The human cost of the ISIS era is difficult to fully account for. Conservative estimates suggest that at least 200,000 people were killed in the conflict to defeat the caliphate — many of them civilians caught in the crossfire of urban warfare in Mosul, Raqqa, and dozens of smaller towns. The true figure is almost certainly higher.
Millions of Iraqis and Syrians were displaced from their homes — some temporarily, many permanently. The Yazidi community suffered a genocide that the UN documented in detail: an estimated 5,000 Yazidi men were executed; between 5,000 and 7,000 Yazidi women and girls were enslaved. Years after the caliphate’s defeat, thousands of Yazidi women and children remained missing, their fate unknown.
The physical infrastructure of Mosul, Raqqa, and other major cities was devastated. Rebuilding has been painfully slow — complicated by political disputes, lack of funding, and the presence of unexploded ordnance across vast areas. Large sections of Mosul’s old city, including the Great Mosque of al-Nuri where al-Baghdadi made his declaration, remain in ruins.
And the conditions that produced ISIS — the sectarian politics of post-invasion Iraq, the collapse of Syrian state authority, the marginalisation of Sunni Arab communities, the poverty and lack of opportunity that makes radicalisation easier — have not been resolved. They have in some cases worsened. The Islamic State as a territorial caliphate may be gone. The forces that summoned it from the ground remain very much present.
This article draws on reporting and analysis from the UN Security Council, the IHS Conflict Monitor, the Soufan Group, the International Crisis Group, and journalists who covered the conflict including those from the New York Times, the BBC, and Vice News. Population estimates are approximate.