The creation of an Islamic caliphate, or empire, has long formed part of al Qaeda's world view, and it is a vision that seems to have unsettled Washington.
But experts say it will remain just a militants' dream.
Before he went into hiding in 2001, Osama bin Laden often talked of deposing Muslim rulers, seen as beholden to Western powers, and abolishing modern state borders to unite all Muslims under a caliphate -- an Islamic state where God's word was law ruled over by a caliph, or "successor" to Prophet Mohammad.
Now al Qaeda militants are talking about setting up a caliphate in west Iraq, and militants calling themselves al Qaeda in Yemen also said recently a caliphate is their goal.
Since September, US President George W. Bush has warned several times that al Qaeda wants to set up a violent, radical Islamic empire based in Iraq, to unite Muslims under one aegis.
Baghdad was once the centre of an Islamic empire that lasted for four centuries -- but experts say the chances of a revival of the ancient Islamic institution are remote.
"Al Qaeda could set up an Islamic state in the west of Iraq, if there is no American army there. But it would be difficult for them to penetrate any other state where there is an army and state apparatus," said Saudi analyst Faris bin Houzam.
"Their big dream is to set up an Islamic state, but there's nothing to suggest it could happen," he added.
One problem they would face is the huge diversity of religious and political systems in the Middle East today, not least between Sunni and Shi'ite Muslims who argue bitterly over the first four caliphs (632-661 AD) to succeed the Prophet.
Shi'ite Iran has developed its own form of Islamic government where supreme authority lies with an Islamic jurist chosen by a body of senior clerics to rule in the place of a disappeared line of descendents of the Prophet.
In Saudi Arabia, the adherents of an austere school of Sunni Islam, known as Wahhabism, give their allegiance to the Saudi family to rule as kings in return for wide latitude to enforce their version of sharia, or Islamic law.
"I can see the whole Arab world falling into sectarian violence, so I can't see this caliphate happening," said London-based anthropologist Madawi al-Rasheed, referring to Sunni-Shi'ite tensions in Iraq and Lebanon.
"This is just part of (al Qaeda's) war of slogans."
Caliphates took on many different forms in the past. Evolving in time across rival Islamic sects that took power in diverse regions, caliphs often claimed descent from the family of the Prophet but rarely wore the mantle of cleric.
The Sunni caliph in Baghdad faced rival claims to lead all Muslims from a Shi'ite caliph in Cairo and a Sunni caliph from a competing dynasty in Muslim Spain.
Lebanese historian Kamal Salibi said the region had already failed to unite under the banner of Arab nationalism after World War Two.
"It didn't work with Arab nationalism, and with pan-Islamism it is working less," he said. "The likelihood that states would give up their sovereignty is now more remote than ever before."
However, the word caliphate, or khilafa in Arabic, still conjures up positive images for most Muslims.
Al-Rasheed said the word is being used more and more in a variety of contexts to signify a morally correct order. It is used by mainstream political parties, Muslim communities and politically interested activists on the Internet.
"With new communications there is a revival of the idea of a caliphate but as a virtual community with no territorial boundaries," she said, pointing to "diaspora" Muslims in Europe.
"Today, in the 21st century, it's a dream of Muslim activists."
Political groups who have indicated a desire to set up a caliphate in their own patch include the Islamist movement in Somalia, Morocco's main opposition group Al Adl Wal Ihssane (Justice and Charity) and Islamist militants in Central Asia.
The spiritual head of Nigeria's 70 million Muslims is referred to by followers as "caliph" and the Muslim region of the country is referred to as the Sokoto Caliphate.
Jemaah Islamiah, the biggest militant organisation in southeast Asia, also dreams of setting up an Islamic state across the region, perhaps using the title "caliphate".
But in the Arab world, Egypt's Muslim Brotherhood has chosen not to focus its efforts on the goal of a caliphate, though scholars have often noted that the seminal Arab Islamist group arose -- perhaps filling a certain psychological need -- only four years after the caliphate was abolished in Istanbul.
The Ottoman empire fashioned itself as the inheritor of the first caliphates centred in Damascus, Baghdad and Cairo until Turkey's secular founder Mustafa Kemal Ataturk formally ended the office in 1924, in what Muslims have often viewed as a British imperial plot.
"For most of the mainstream and less mainstream political parties of political Islam, the borders of the contemporary state have been accepted," said As'ad AbuKhalil from Lebanon, who teaches politics at the US California State University.
"There is absolutely no credence to the notion that the quest for the caliphate is the overriding goal of the Islamist movement in the region."