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Ridiculed at first, the new power which has seized a third of Iraq and triggered the first American air strikes since the U.S. troop withdrawal in 2011 - has carved itself a powerful and possibly lasting presence in the Middle East.
The bombing of fighters of the Sunni Islamic State is unlikely to turn around Iraq and its fragmented condition has given the self-proclaimed caliphate the opportunity to establish a hub of jihadism in the heart of the Arab world.
To confront the Islamic State storming through the villages of eastern Syria and western Iraq, an international coalition sanctioned by the United Nations would need to be set up, analysts in and outside the Gulf region said.
The jihadist army, whose ambition for a cross-border caliphate between the Euphrates and the Tigris rivers was not initially taken seriously by their opponents, is now brimming with confidence, emboldened by blood and treasure.
The warriors of the new caliphate are exploiting sectarian and tribal faultlines in Arab society, petrifying communities into submission and exploiting the reluctance of Washington and the West to intervene more robustly in the civil war in Syria.
Unlike Osama bin Laden's al Qaeda, which set its sights on destroying the West, the Islamic State has territorial goals, aims to set up social structures and rages against the Sykes-Picot agreement of 1916 between Britain and France that split the Ottoman empire and carved borders across the Arab lands.
President Barack Obama's decision to step back into the Iraq quagmire nearly three years after withdrawing U.S. troops, with limited air strikes in the past few days against the Islamic State, arises in part because of inertia over Syria.
A failure to arm the mainstream, mostly Sunni, rebellion against Bashar al-Assad's authoritarian rule opened space for the Islamic State, which has now surged back into a broken Iraq, raising its black flag in town after town, the analysts said.
Almost a year ago, in a last-minute change of mind, Obama decided against bombing Assad amid accusations of nerve gas attacks on rebel enclaves. That decision, many believe, has proved costly both in Syria and in neighbouring Iraq.
It reinvigorated Assad, helped in the quashing of Syria's moderate rebels and empowered the militant Islamists who became a recruiting magnet for disenchanted Sunnis in Syria and Iraq.
Well financed and armed, IS insurgents have captured large swathes of territory in a summer offensive, as the Iraqi army - and now