Saudi Arabia has announced it is sending 30,000 soldiers to monitor the border with Iraq, amid fears over the spread of Islamic State
Saudi Arabia has sent 30,000 troops to reinforce its long northern desert border after Iraqi troops withdrew from the other side, according to reports
Fighters from Islamic State, the jihadist group, and its allies have already seized frontier posts on Iraq's western borders with Syria and Jordan. The southern border with Saudi Arabia, which regards itself as vulnerable to the threat of jihadism, is more than 500 miles long.
Large parts of it are with Anbar province, the centre of Islamic State power in Iraq and now almost entirely under the control either of the group itself or of Sunni Arab tribes that have allied with it.
The last serious incursion into Saudi Arabia also came from Iraq, after Saddam Hussein invaded Kuwait in 1990. Iraqi forces were eventually repelled from the Saudi town of Khafji, the following January, but only after they were attacked from the air by American jets.
Since then, the border has been relatively stable and far more concern and attention has been paid to the southern border with Yemen, which is mountainous and hard to control, and where there have been two separate, long-running insurgencies, one jihadist, and one led by a Shia breakaway movement.
However, the al-Arabiya news channel, which is private but close to the Saudi government, said Riyadh had ordered the troops to the border after the Iraqi side appeared undefended.
An Arabiya sister channel showed an interview with an Iraqi army officer relocated to near Karbala, in southern Iraq, saying he did not know why the order had been given to withdraw.
Saudi Arabia is often accused of fuelling the Islamic State insurgency, either directly with money or at least by its support for the uprising in Syria. It denies funding the group, most of whose finances comes from its own business and extortion operations, though it has some private backers in the Gulf, and Riyadh is often publicly threatened by the group.
Riyadh instead blames the government of Nouri al-Maliki for its alienation of the Sunni Arab minority in Iraq. Saddam Hussein was Sunni, and since he was ejected from office by the allied invasion of 2003 the Shia majority have been able to seize most of the levers of power.
Attempts by Mr Maliki to form a new, cross-sectarian government on Monday failed, and there are continuing calls for him to step down. The Sunni tribes allied to the Islamic State have vowed not to reconsider their position until that happens.
In the meantime, the group has unfettered power over much of western and northern Iraq, though it has showed no signs of wanting to cross the border into either Jordan or Saudi Arabia, which would normally be considered "red lines" for intervention by the countries' main international backer, the United States.
Its growing dominance means that fighters in Syria once loyal to its rivals have now started offering "bayah" to the Islamic State's leader, Abu Bakr al-Bahdadi, or Caliph Ibrahim as he is now styled following the group's declaration of a Muslim Caliphate.
A group of fighters from tribes close to Jabhat al-Nusra, al-Qaeda's official affiliate in Syria, which has been fighting the Islamic State, have declared allegiance in the town of Sheheil, according to the Syrian Observatory for Human Rights.
