BEFORE the events of 9/11 few seemed aware of a burgeoning radical Islam. How many had heard of the Wahhabi sect, whose intolerant ideology has so hijacked the way the west now views Muslims? In this informative, persuasive book, Charles Allen shows how we should have been paying more attention, because our present situation has its roots as far back as the 18th century.

Allen painstakingly follows two threads: the spread of Wahhabism through Arabia, with its links to the Ibn Saud family who unified a country and nabbed a kingdom, and the parallel trajectory of various reformist movements in northern India. These led ultimately to the formation of the Deoband religious college in 1866, which promoted a fundamentalism no less extreme. It became the model for Pakistan's 10,000 madrassahs.

At the end of the 20th century the confluence of these two streams saw the emergence of the backward-looking Taliban . . . many of whose foot soldiers were madrassah-educated . . . and the globally ambitious al-Qaeda.

What's clear in this fascinating narrative is that the call for a purer form of Islam was never solely a religious urge. Muslim militancy became coupled in British India with a political desire to do away with the humiliations of the Raj, and on the part of the Pathans and Afghans with resistance to an invasion that threatened their religion.

In Arabia, the canny Ibn Sauds wanted power and exploited the wish of desert tribes and zealots to break away from Ottoman rule.

The ideas behind Wahhabism date back 700 years but it was the Sunni Muslim Muhammad ibn Abd al-Wahhab who set the ball rolling in the 1740s. In his Book Of Unity this "provincial bumpkin with little access to Islamic scholarship" interpreted the Koran in a literalist way, rejecting any innovation and committing to the violent destruction of polytheists, infidels and unbelievers. These included not just Christians but Shias (the largest minority sect), Sufis, and mainstream Sunnis. With its selective focus, "Al-Wahhab's Islam effectively sidelined the Quran's central message of charity, tolerance, forgiveness and mercy".

Ibn Abd al-Wahhab, disowned as a heretic by his own family, formed a dynastic alliance with Muhammad ibn Saud, a tribal chief in Nejd.

Ibn Saud's son married Ibn Abd alWahhab's daughter and was responsible for introducing the spear-carrying Bedouin to firearms. Under him, the cult of martyrdom, originally associated with Shias, took off. Aggressive tactics . . . desecrating Shia shrines in Persia, mutilating opponents, closing down the pilgrimage routes . . . proved ruthlessly efficient. By aligning themselves with the British against the Turks in the first world war, the House of Saud by the 1920s had Arabia under its Wahhabi thumb.

Muslim despair on the sub-continent, meanwhile, resulted from the dismantling of Mughal kingdoms and the emasculating policies of the empire-building British. But those demanding a return to an uncompromising Islam didn't see themselves as Wahhabis. The hub of these "Hindustani Fanatics" lay in Sittana, a fortified village on the Mahabun Mountain. Relying on Pathan hospitality on the hostile North-West Frontier, they fashioned themselves as mujahedeen (holy warriors) whose purpose was also to undermine British rule.

Many of the "fanatics" were poor and illiterate, but after the Indian Mutiny of 1857, deserting Britishtrained NCOs joined them.

While the Arab Wahhabis relied on raiding parties and terror, their Asian counterparts operated a number of sophisticated guerrilla movements. They understood the importance of taking their message to the streets, making use of printing as early as 1821 to circulate the call for jihad. They wrote in Urdu, the language of the masses. Those organising the supplies, money and men from the recruiting centre in Patna were embedded in the British Imperial system: a tax collector, a postmaster, a respected mullah.

British complacency missed several opportunities to stamp out the frontier jihad. Were the "Fanatics" really behind the Indian Mutiny of 1857? It's possible. But what is more certain is that British-Muslim relations deteriorated. The murders of a British chief justice and a viceroy by Muslims 15 years later did nothing to help. The Muslim community, seeing itself as victimised, turned increasingly from secular leaders to spiritual ones. It was in this climate that Deobandism flourished.

Oil transformed Saudi Arabia into a powerful force in the 1970s, allowing the Wahhabi message to be spread with the welcome boost of petro-dollars. Under the military dictator General Zia-ul Haq, Deoband-trained idealists filled army, civil service and secret service posts. When Soviet forces invaded Afghanistan, a recruitment centre for foreign volunteer fighters was established in Peshawar and funded by Saudi cash. From this office, according to Allen, the seeds of international jihad took root as Al-Qa'eda.

Allen keeps a tight grip on complicated, often nebulous material.

Rendering graphic accounts of hard-fought battles like the Malakand campaign, he also introduces some colourful characters.

Harry St John Philby, father of Kim, was a political agent who converted to Ibn Saud's cause and betrayed the British government.

He clearly taught his son something about flexible loyalties.

As he was unable to decipher original sources, Allen's perspective is largely drawn from British historical accounts, which makes his view perhaps one-sided. But if he is antipathetic to Wahhabism, he is also critical of western disregard for injustices in the Muslim world.

Remove these grievances, he suggests, and support for the extremists would vaporise. Moderate Muslims could finally claim back Islam from God's terrorists.