Seeing all the commemorations of the First World War centennial, many might ask what relevance such distant struggles can have for the modern world. Why do they matter? But if they look at the world’s most dangerous storm centres today – in Iraq and Syria, across the Middle East and South Asia – they will get their answer. In these regions, as in so much of the world, the First World War created our reality.

Out of the political ferment immediately following the war came the most significant modern movements within Islam, including the most alarming forms of Islamist extremism. So did the separatism that eventually gave birth to the Islamic state of Pakistan and the heady new currents transforming Iranian Shi’ism. From this mayhem also emerged what would become the Saudi state, dominating the holy places and rooted in strictly traditional notions of faith.

When the war started, the Ottoman Empire was the only remaining Islamic nation that could even loosely claim Great Power status. Its rulers knew, however, that Russia and other European states planned to conquer and partition it. Seizing at a last desperate hope, the Ottomans allied with Germany. When they lost the war in 1918, the Empire dissolved. Crucially, in 1924, the new Turkey abolished the office of the Caliphate, which at that point dated back almost 1,300 years. That marked a trauma that the Islamic world is still fighting to come to terms with.

How could Islam survive without an explicit, material symbol at its heart? The mere threat of abolition galvanized a previously quiet Islamic population in what was then British India. Previously, Muslims had been content to accept a drift to independence under Gandhi’s Hindu-dominated Congress party. Now, though, theKhilafat (Caliphate) movement demanded Muslim rights, and calls for a Muslim nation were not far off. That agitation was the origin of the schism that led to India’s bloody partition in 1947, and the birth of Pakistan.

How to live without a Caliph? Later Muslim movements sought various ways of living in such a puzzling and barren world, and the solutions they found were very diverse: neo-orthodoxy and neo-fundamentalism, liberal modernization and nationalism, charismatic leadership and millenarianism. All modern Islamist movements stem from these debates, and following intense activism, Egypt’s Muslim Brotherhood was formed in 1928.

One of the founders of modern Islamism was Maulana Mawdudi, who offered a comprehensive vision of a fundamentalist Islam that could confront the modern world. Although Mawdudi was born in 1903, he was already involved in journalism and political activism before the end of the Great War, and by the start of the 1920s he was participating in the fierce controversies then dividing Muslim thinkers in the age of the Khilafat. In 1941, he founded the Jamaat-e-Islami, the ancestor of all the main Islamist movements in Pakistan and South Asia, including the most notorious terrorist groups.

For many Muslims, resurgent religious loyalties trumped national or imperial allegiances. Armed Islamic resistance movements challenged most of the colonial powers in the post-war years, and some of those wars blazed for a decade after the fighting ended on the western front. That wave of armed upsurges would be instantly recognizable to American strategists today, who are so accustomed to the idea of a turbulent Arc of Crisis stretching from North Africa through the Middle East and into Central and Southern Asia.

Just to give an idea of the scale of the movements, between 1919 and 1925 Britain’s newly founded Royal Air Force saw action against Muslim rebels and enemy regimes in Somalia, Afghanistan, Waziristan and Iraq. Throughout the 1920s, the Basmachi revolt fielded tens of thousands of guerrillas against the Soviet Union, fighting on behalf of an autonomous shari’a state and operating across most of Soviet Central Asia. And for Muslim insurgents, these struggles bore the holy sanction of jihad warfare. One rebel chieftain in the Caucasus promised to hang all who wrote from left to right.

Most of the revolts of these years grew directly out of wartime agitation, and some give a powerful sense of deja vu. Before the war, the region we now call Iraq was a loosely-connected region that included three wilayat or provinces – namely Mosul, Baghdad and Basra – all very diverse religiously. With the end of Ottoman power, the British brought the three together, hoping to dominate an area that was already a prime oil producer. But resistance swelled. By 1920, the British were meeting growing resistance from both Sunni and Shi’a populations, as former Ottoman officers buttressed the opposition. Leading Shi’a clergy and ayatollahs then issued fatwas proclaiming the illegitimacy of British rule and calling for rebellion.

Wide-ranging revolts across the huge territory became something close to a national insurrection in 1920, which the British defeated only by deploying the latest technology of air power and poison gas. After the revolt, the British ruled the mandate through their old Arab ally Faisal, who received the kingship in 1921. Never, though, would any later regime succeed in uniting those radically diverse forces that European empires had squeezed together into one unhappy union. Event of recent days show that the Iraq problem still has no clear solution.

One lasting legacy of the Iraqi conflict was the shift of Shi’a religious authority from the city of Najaf. The beneficiary was the emerging intellectual centre of Qom, in Persia, the nursery of generations of later ayatollahs. Although the school’s new heads disdained political activism, they could not fail to see how quickly and easily secular regimes had crumbled over the past decade, leaving clergy as the voices of moral authority and the defenders of ordinary believers. In 1921, the nineteen-year-old Ruhollah Khomeini was already a student at Qom, long before his later elevation to the prestigious rank of ayatollah. Like his counterparts in Egypt and British India, Khomeini grew up seeking a world order founded on a primitive vision of authentically Islamic religious authority.

For Muslims, the Great War changed everything. Modern political leaders look nervously at the power of radical Islam and especially those variants of strict fundamentalism that dream of returning to a pristine Islamic order, with states founded on strict interpretation of Islamic law, shari’a. Terms such as jihad provoke nightmares in Western political discourse. All these concepts were well known a century ago, but it was the crisis during and immediately following the war that brought them into the modern world.

What we think of today as modern Islam – assertive, self-confident and aggressively sectarian – is the product of the worldwide tumult associated with the Great War. Islam certainly existed in 1900, but the modern Islamic world order was new in 1918.

Philip Jenkins is Distinguished Professor of History and Co-Director of the Program on Historical Studies of Religion at theInstitute for Studies of Religion, Baylor University. His most recent book is The Great and Holy War: How World War I Became a Religious Crusade.