Middle East analyst Juan Cole has a long charge sheet including: the Bush administration, Saudi Arabia, the Iraqi army, and Nouri al-Maliki. But he says the roots of the problem go right back to the "shameful European imperial scramble for the Middle East during and after WWI".
He writes:
Integrating Mosul into British Iraq, over which London placed Faisal b. Hussein as imported king after the French unceremoniously ushered him from Damascus, allowed the British to depend on the old Ottoman Sunni elite, including former Ottoman officers trained in what is now Turkey. This strategy marginalized the Shia south, full of poor peasants and small towns, which, if they gave the British trouble, were simply bombed by the RAF. (Iraq under British rule was intensively aerially bombed for a decade and RAF officers were so embarrassed by these proceedings that they worried about the British public finding out.)
To rule fractious Syria, the French (1920-1943) appealed to religious minorities such as the Alawites and Christians to divide and rule; Alawite peasants were willing to join the colonial military as proud Damascene Sunni families largely were not, but when the age of military dictatorships overtook the postcolonial Middle east, the Alawites were in a good position to take over Syria, which they definitively did in 1970 ...
Nouri al-Maliki can only get Iraq back by allying with nationalist Sunnis in the north. Otherwise, for him simply brutally to occupy the city with Shiite troops and artillery and aerial bombing will make him look like his neighbor, Bashar al-Assad.
see 7.26pm AEST : http://www.theguardian.com/world/2014/jun/12/crisis-in-iraq-insurgents-take-major-cities-live-blog#block-53997030e4b00360d270f2df