It began dramatically on Sept. 11, 2001. Our century is characterized by a lethal theological war in the house of Islam, with brutal consequences for the whole world, whether it be lower Manhattan or northern Iraq.

Centuries are not exactly 100 years long. The late British historian Eric Hobsbawm proposed a more persuasive division of history with his “long 19th century,” which began with the French Revolution in 1789 and lasted until the Great War laid waste to the empires and their balances of power. That was followed by the “short 20th century,” inaugurated by the Bolshevik Revolution in 1917 and concluded by the dissolution of the Soviet empire in 1989, and the erasure of the Soviet Union itself from the map in 1991.

What followed in the 1990s was a short “holiday from history,” as George F. Will put it on Sept. 12, 2001. A new century had begun that day, and whether it will be long or short remains to be seen. Last week, I wrote about how the settlement of the Great War eliminated Islam’s geopolitical expression on the global stage and divided up historic Muslim lands — including, in the Islamic view, the ancient capital of the world’s religious identity, Jerusalem — among European powers. While the Cold War occupied Western energies during the short 20th century, the Islamic world roiled with the question of what would replace the Ottoman empire as Islam’s geopolitical expression.

One answer was given in 1979, from Shia Iran, where an Islamic Republic was established — state power at the service of Islam in a theocracy. Another answer was incubating in the Sunni world, which sought influence outside of the state system, even as it enjoyed the financial support of the Sunni powers. This latter option took the path of guerrilla war and terror, making itself manifest with al-Qaeda’s assault on the twin towers in New York.

So the 21st century opened with two options facing the Islamic world. On one hand, an Islamic jihad pursued through both state theocracy and non-state actors employing terrorism. These options overlapped both the Sunni and Shia worlds, as Hamas, Hezbollah, Al-Nusra and now ISIS demonstrate. On the other hand, we see the continuation of monarchies and military rulers which are supportive of Islam but not wholly animated by it. The Arab Spring marked the rebellion of the former against the latter. The turn of Turkey away from its aggressive secularism toward a more explicitly Islamic posture is another indication of the rise of the jihadist view over the idea of a secular state.

It is not a war between religions, and not a war between Islam and Christianity. It is a theological war in Islam that has lethal consequences for all those it touches, including non-Muslims. The West was reluctant to concede that our century would be defined by a theological war within Islam. And so President George W. Bush spoke of a “war on terror”, which is a technique, not an idea or a movement. President Obama thought it was principally about America, and so his sweet words spoken in Cairo could make it all go away.

One of the few to see these developments clearly — more clearly than either his predecessor or his successor it should be noted — was Pope Benedict XVI, who called the world’s attention to this Islamic theological-political crisis at Regensburg, Germany, in 2006. Many sought to evade the pointed questions he posed, but they did resonate in the Islamic world, where Muslims are the first to live with the consequences of jihadism.

All those evasions are no longer tenable. The unspeakable brutality taking place in ISIS-controlled Iraq — beheadings, crucifixions, killing of children, selling of women, mass graves, expulsions of entire populations — offers a clear picture of one option currently on offer. It is powerful and growing, manifesting itself in both Sunni and Shia forms in parts of the Islamic world. Its defeat will require from within Islam a century-long struggle involving theology, politics, social and cultural reform and sheer military power.

The task of the West is to protect its populations from the spillover effects when the jihadi cauldron overflows. Israel, as the most proximate Western nation, has learned this first, which is why it is reluctant to seek allies amid warring Islamic factions, and resigns itself instead to repeated small wars on its borders.

When the 21st century began with the carnage of September 11, it was tempting to ignore that a bloody new century had begun. Thirteen years into the jihadist century, it can no longer be ignored.