Regional tensions: How religious and military divides shape the Middle East
At the heart of the terrifying meltdown in Iraq is the centuries-old hatred between two Muslim ideologies: Sunni and Shia.
The deadly power struggle between these two rival versions of the same faith has flared into life as Sunnis in the extremist terror group Islamic State of Iraq and the Levant (ISIS) advance on Baghdad, where flailing prime minister Nouri al-Maliki - who is Shia - begged his parliament to declare a state of emergency.
It is a battle being watched with trepidation throughout the Middle East, where the escalation of the traditional Sunni/Shia conflict threatens governments and national borders.
Already, ISIS has effectively established its own nation state - or Islamic caliphate - which spreads across the north of Syria and Iraq, taking no heed of the border between the countries.
Threatening: Men pose with automatic rifles and a stationary machine gun, with the ISIS flag propped up behind them
Its extraordinary success could not have been achieved without the tacit support of ordinary Sunni people in the areas it has conquered.
The Sunnis in Mosul regarded the Shia-dominated army from the south of the country as an occupying force and were only too pleased to see the back of them.
True, these people are terrified of the brutal ideology of ISIS, which specialises in amputations and crucifixions for those who do not subscribe to its fundamentalist creed.
But for now, their hatred of al-Maliki’s authoritarian government, which treats them as a lower caste, outweighs those fears.
To add to the tribal tensions in Iraq, the country’s north-eastern Kurdish population - who were persecuted by Saddam Hussein and gassed in their thousands - have established what is, in effect, their own independent state in the north of the country.
Their force of 250,000 crack Peshmerga militia - who have just taken the oil-rich city of Kirkuk - could defeat ISIS, but they are in dispute with al-Maliki over oil revenues and are in no mood to help.
Warlike: The Kurdish Peshmerga armed forces, pictured yesterday in Kirkuk, Iraq, could defeat ISIS, but are in no mood to
Warriors: The Kurdish people have managed to establish their own state - after years of persecution under Saddam Hussein
Meanwhile, across the Middle East, Sunni and Shia rivalries are festering like open sores. Of the world’s 1.6billion Muslims, the vast majority are Sunnis; Shias comprise 10 to 15 per cent - two hundred million people.
Egypt, Turkey, Bahrain and Saudi Arabia are Sunni. In Bahrain and Saudi Arabia, the ruling Sunni treat Shia as second-class citizens.
The Shia are concentrated in Iran, southern Iraq and Lebanon. And despite being in the minority in Syria, they are powerful there, too: President Bashar Assad’s ruling party belong to a Shia sect called the Alawites.
Once you understand the Sunni/Shia divide, you can make sense of the rivalries in the Middle East. It explains why Sunni rebels - backed by the predominantly Sunni powers, ranging from Turkey to Saudi Arabia and the smaller Gulf states - are determined to fight Assad’s Shia-dominated army to the death.
And why Lebanese Hizbollah militias (Shia) are fighting for Assad, under the command of Revolutionary Guards officers from Iran (also Shia).
The most extraordinary fact in all this is that the conflict goes back to the seventh century and centres on a dispute over who should succeed Islam’s founder Prophet Muhammad after he died in 632 AD.
The largest group (Sunnis) wanted traditional tribal elders to decide upon the best person; the name Sunni comes from Ahl al-Sunna, meaning the people of tradition.
A minority (Shia) wanted a blood relative of the Prophet, and this clash grew violent when Ali, the Prophet’s cousin and son-in-law, became the fourth caliph - an office that fuses political and religious power. Shia derive their name from shiaat Ali or followers of Ali.
ISIS fire heavy machine guns during fighting in the northern Iraqi city of Samarra - today the Islamic State has issued a triumphalist statement declaring that it would start implementing its strict version of Shariah law in Mosul and other regions it had overrun
Dozens of members of a police special forces battalion were paraded before a crowd in the Iraqi city of Tikrit on Thursday after they were captured by fighters who overran their base. Militants have set up military councils to run the towns they captured, residents said.
During the years of Empire, these divisions were muted as Sunni and Shia united against the colonial rulers, who took little account of tribal rivalry when they arbitrarily created new countries such as Iraq, a concoction dreamed up by Britain and France in 1921 after the fall of the Turkish Ottoman Empire.
Two former Turkish territories were handed to princes in the Hashemite family. Prince Feisal, a friend of Lawrence of Arabia, would become king of a new country called Iraq. His brother, Prince Abdullah, would rule Transjordan - now Jordan.
Authoritarian rulers - Saddam Hussein, President Assad and Colonel Gaddafi in Libya - ruthlessly kept a lid on the religious rivalry.
But with their removal, the divisions have exploded throughout the Middle East and beyond.
This is why extreme fundamentalist Sunnis who wish to restore the medieval caliphate are on the march.
Given the chance, they would kill all Shia as heretics - along with Jews and Christians - and sweep away corrupt and ‘faithless’ rulers in the region, from Jordan’s King Abdullah to King Abdullah of Saudi Arabia.
This explains why the very Gulf rulers who covertly back them - if only in an attempt to buy themselves peace and encourage them to leave their shores for jihadist missions abroad - are even more terrified than the rest of us.