Under normal circumstances, Iran, the most powerful Shia country in the region, treats all religious Iraqi Shia parties equally.
In Iraq, that meant one of two targets – a seat of Shia power or, even better, a defining religious symbol.
In doing so, the assailants maintain an ancient tradition: the Sunni caliph Mansur founded Baghdad in 672 and left a crypt full of Shia corpses below his palace.
Iraq's Sunnis, disenfranchised by the overthrow of their patron, Saddam Hussein, were taking the fight to US forces – and starting to turn their guns towards the beneficiaries of Hussein's overthrow, the country's majority Shia population.
In the opening scene, a weeping Charlie (Shia LaBeouf) observes his dying mother (Melissa Leo) being taken off life support in a Chicago hospital.
He had been galvanised into militancy as a young man by an American occupation that he and many like him believed was trying to impose a power shift in Iraq, favouring the country's larger Shia population at the expense of the dominant Sunnis.
And it is the Saudis who now, in improbable alliance with Binyamin Netanyahu's Israel, lobby most forcefully against any American nuclear deal, or broader western rapprochement, with Shia Iran, their sworn enemy.
Muslim dismay at Tony Blair's co-invasion of Iraq in 2003 did not sway that decision, though it could be argued that anti-war sentiment inhibited proper post-invasion occupation planning and encouraged the Sunni insurrection against both invaders and Shia Muslims.
A carefully-managed Manama conference on the "dialogue of civilisations" included Christian and Jewish delegations but excluded mainstream Shia or opposition clerics.
In Shia Islam, the hidden imam is predicted to reappear in Mecca and herald a new dawn of justice after the world has been torn asunder by violence and oppression.
