As with almost any religion, just about any generalization about Islam is likely to be wrong. One can say with certainty that Muslims believe in one God and Muhammad as the Prophet; daily ritual prayer; almsgiving; fasting during Ramadan; and a pilgrimage to Mecca once in one's lifetime.
But on many matters, Islam's different schools and traditions often disagree. The biggest argument centers on who should have been Muhammad's successor. That still divides Sunni Muslims (the majority) from Shiite Muslims.
Many Muslims are trying to shape Islam for the 21st century. But a profound internal conflict is raging within the religion, and the United States felt its effects on Sept. 11, 2001. It is a conflict waged by a relatively small number of Muslims who believe they are the true interpreters of Islam - and that those who do not share their views are heretics and apostates who must be destroyed. They want to return to an imagined golden era of Islamic rule, and see modern governments and the West, especially the US, as obstacles.
The Afghan Taliban and Al Qaeda are manifestations of this thinking, which derives largely from Wahhabism, an 18th-century puritanical interpretation of Sunni Islam practiced in Saudi Arabia and officially supported - to the exclusion of other forms of Islam, let alone any other religion - by the Saudi royal family.
The Saudi government and individuals began aggressively exporting Wahhabism after the Shiite leader Ayatollah Khomeini came to power in Iran in 1979. Money and missionaries promoting Wahhabism flowed worldwide, giving rise to mosques and schools that often preach hatred toward adherents of other religions and of differing views of Islam.
Wahhabist influence has reached its zenith outside the Arabian peninsula in Afghanistan and Pakistan, especially among ethnic Pashtuns resisting the Soviet occupation of Afghanistan in the 1980s. The Afghan Taliban who sheltered Al Qaeda were themselves trained in Pakistan, where so-called Islamic schools are still churning out radicalized fighters. Others may be hiding Osama bin Laden and his close associates. Others murder Pakistani Christians or Shiites, as happened last week when attackers killed 53 Shiites in a Quetta mosque.
But it shows up elsewhere, as well: in former Soviet republics of Central Asia; in Indonesia, in attacks on Christians and in last year's Bali bombing; and in northern Nigeria, where a woman was sentenced by an Islamic court to be stoned for having a baby out of wedlock.
Almost every religion has been misused to produce similar phenomena in its history. But few religious-based groups have advocated mass murder and violence as Al Qaeda does, and Wahhabist intolerance prepared the ground for that.
(It's important not to lump all "fundamentalist" Muslim movements in the Wahhabist or Al Qaeda camp. Many are more tied to local politics or ethnic grievances.)
Now that the US and the rest of the Western world have been drawn by terrorists into this inter-Muslim conflict, the difficulty is to determine how to influence the outcome. Should the US, in effect, support one version of a religion over another?
In the end, Muslims must sort this out among themselves. But the US can strengthen the forces of moderation by supporting economic development, tolerance, and human rights in Muslim nations. It can work to end the Israeli-Palestinian dispute, which Al Qaeda and others exploit to gain recruits. It can increase educational and exchange programs, bringing Muslim leaders to the US to learn how people of different religious traditions can live together. It can discuss these issues sensitively in media broadcasts. And it can continue to press the Saudi government to foster tolerance within Saudi Arabia itself and to crack down on those funding hatred elsewhere.
US nation-building success in Iraq and Afghanistan will set a powerful example. Groups such as the Taliban and Al Qaeda know this and will try to sabotage US and allied efforts.
These are not simple matters of foreign aid or naive do-goodism. The security of Americans, Britons, French, and many others - not to mention the well-being of more than 1 billion Muslims - depends on how this sectarian struggle plays out.
It may be the most important part of the war on terrorism.