In 2006, the investigative reporter Jeff Stein concluded a series of interviews with senior US counterterrorism officials by asking the same simple question: "Do you know the difference between a Sunni and a Shia?" He was startled by the responses. "One's in one location, another's in another location," said Congressman Terry Everett, a member of the House intelligence committee, before conceding: "No, to be honest with you, I don't know." When Stein asked Congressman Silvestre Reyes, chair of the House intelligence committee, whether al-Qaeda was Sunni or Shia, he answered: "Predominantly - probably Shia."
Wrong. Al-Qaeda is a Sunni Muslim terrorist group that considers Shia Muslims to be heretics. In Stein's words, "Al-Qaeda's Sunni roots account for its very existence." Osama Bin Laden and his ilk are the ideological offshoots of the hardline Salafi, Deobandi and Wahhabi brands of Sunni Islam that today dominate Saudi Arabia, the Gulf countries, Afghanistan and large parts of Pakistan. And the leading thinkers behind modern Islamist movements such as al-Qaeda are all Sunnis: Abul-Ala Maududi, Hassan al-Banna, Sayyid Qutb, Abdullah Azzam. So is it too much to expect US officials to understand the implications of the Sunni-Shia divide? Perhaps. As the former senator Trent Lott confessed: "They all look the same to me."
Shias and Sunnis are indeed united on core beliefs - in God, the Prophet Muhammad, the Quran - but there remain major doctrinal, jurisprudential and political differences between the two sects. The split occurred in the wake of Muhammad's death, in AD632. Some Muslims supported the right of the Prophet's family - specifically his cousin Ali - to succeed him, but more endorsed the Prophet's companion Abu Bakr, who then took power. The latter became the Sunnis (or followers of the sunnah, the lifestyle, of the Prophet), the former the Shias (or partisans of Ali).
Ignorance of these two sects of Islam, which together comprise a fifth of humanity, extends far beyond Washington, DC. I remember once, when I worked in television, a senior executive claiming he'd made a bet with his wife that Shias were the bigger group and Sunnis the minority. "You're Shia, aren't you?" he asked me. "So are you guys the majority? Am I right?" I had to break the bad news to him.
According to the Pew Forum, of the total Muslim population across the world, 10-13 per cent (or roughly 200 million people) are Shias and between 87 and 90 per cent are Sunnis. Most Shias (68-80 per cent) live in just four countries: Iran, Pakistan, India and Iraq. And there are only four Shia-majority Muslim nations: Iran, Iraq, Azerbaijan and Bahrain.
But confusion reigns. At the Iraq inquiry, Tony Blair - now head of the Tony Blair Faith Foundation - linked Shia Iran to the "extreme and . . . misguided view about Islam" promoted by Sunni groups such as al-Qaeda, before claiming that Iran had been "supporting" al-Qaeda attacks inside Iraq. It seems Blair is ignorant of al-Qaeda's view of Iraqi Shias. In 2004, Abu Musab al-Zarqawi (since killed by US forces) wrote: "They [the Shias] are the insurmountable obstacle, the lurking snake, the crafty and malicious scorpion, the spying enemy and the penetrating venom." Shia Islam, he concluded, "is the looming danger and the true challenge".
Blair's war in Iraq served to exacerbate the age-old divide between Sunnis and Shias - leading to Iraqi elections, for example, organised on sectarian lines - and fed the insecurities of the Sunni-dominated Muslim world. The al-Qaeda view of Shia Islam has since seeped into the Sunni mainstream. Wahhabi Saudi Arabia has long considered Shias to be "disbelievers", but now newspapers in secular Egypt warn of a Shia threat, and last year Jordan put six Shias on trial for "promoting" Shia ideology.
Sunni leaders point to the threat of Shia extremism from Iran. Iran's ayatollahs fund Hamas and Hezbollah - both proscribed terrorist groups in the west. But contemporary Iran is qualitatively different from Afghanistan under the Sunni Taliban, with its cultural barbarism. Middle East experts agree that the Shia institution of ayatollahs, or scholars, interpreting and updating Islamic law, has acted as a theological bulwark against al-Qaeda's distinctive brand of nihilistic and apocalyptic violence.
Nor should Shia Iran, and the challenge of its nuclear programme, be confused with the terrorist threat to the west from al-Qaeda-inspired Sunni extremism. Not one of the 111 Muslims imprisoned in Britain on terrorism offences is believed to be a Shia. Most Sunnis, like Shias, reject extremism and violence, but in the context of terrorism, it is important to understand the exact nature, and ideology, of the threat. To lump all Muslims together is self-defeating.
Vali Nasr, a professor at the US Naval Postgraduate School, argues that wars within Islam between Sunnis and Shias "will shape the future" of the Muslim world and, in particular, the Middle East. "Ultimately," he predicts, "the character of the region will be decided in the crucible of Shia revival and the Sunni response to it." Perhaps he overstates the point. But anyone who claims to care about the future of the Muslim world needs to understand this 1,400-year-old split at the heart of Islam.