But Ayman al-Zawahiri, an Egyptian doctor considered to be al-Qaeda’s leading strategic planner, also cautioned Abu Musab al-Zarqawi, the Jordanian-born terrorist chief in Iraq, that slaughtering Shia civilians could undermine their grand design by turning popular Muslim opinion against them.

Al-Qaeda’s ambitious vision is revealed in a 6,000-word letter, seized by US troops during a counter-terrorism operation in Iraq, that offers a unique insight into the organisation’s strategy. The letter, which the Bush Administration has just posted on the internet (www.dni.gov), also reveals a schism in al-Qaeda, with even al-Zawahiri questioning the brutality of al-Qaeda’s “emir” in Iraq.

Al-Zawahiri, in a sign that al-Qaeda’s old guard — which organised the September 11 attacks on the United States — may be losing ground to younger terrorists in Iraq, pleads with his protégé for money even as he urges him to reign in his murderous urges. US officials said it was “absolutely certain” the letter was meant for Zarqawi, though he is mentioned only obliquely, and they vouched for its authenticity.

Zawahiri and Zarqawi are clearly at one in their desire to establish the Prophet Muhammad’s 7th-century vision of a kingdom of God on Earth, and want it to end in an apocalyptic showdown with Israel. Al-Zawahiri sets out for the first time a step-by-step plan to re-create Islam’s medieval caliphate, starting with the expulsion of US forces from Iraq, establishing an Islamist emirate in Baghdad. and then spreading jihad into nearby secular states such as Jordan, Syria, Egypt, Saudi Arabia, Lebanon and Kuwait.

The final stage would be a showdown with Israel “because Israel was established only to challenge any new Islamic entity”, he writes.

The letter emphasises that the Mujahidin’s mission must not end with the expulsion of the Americans from Iraq “and then lay down their weapons and silence the fighting zeal. We will return to having the secularists and traitors holding sway over us. Instead the ongoing mission is to establish an Islamic state . . . until the Hour of Resurrection.”

Al-Zawahiri clearly fears that al-Zarqawi’s constant attacks on Shia mosques in Iraq and his beheading of hostages on video have sickened many Arabs who once showed sympathy for al-Qaeda.

The older terrorist emphasi- ses that the group would need a more political stance to take advantage of a power vacuum when the American military left Iraq.

“Things may develop faster than we imagine,” al-Zawahiri writes. “The aftermath of the collapse of American power in Vietnam — and how they ran and left their agents — is noteworthy. Because of that, we must be starting now, before events overtake us.”

Al-Zawahiri suggests al-Zarqawi begins to transfer some power from his foreign insurgents — “the Islamic Mujahidin” — to Iraqi guerillas to avoid a backlash against foreign interference in Iraq’s affairs.

He recalls how the Taleban in Afghanistan restricted their leadership to a handful of fanatical students based in Kandahar, and how quickly their support vanished when the US Army and its Afghan allies attacked in 2001.

“In the absence of popular support, the Islamic Mujahidin movement would be crushed in the shadows, far from the masses who are distracted or fearful,” al-Zawahiri says.

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