Donald Rumsfeld may have been right when he commented on the dangers of "unknown unknowns", how one action can have so many unpredictable results and reactions.
In Iraq what was predictable was that once the US stopped fighting and started paying members of the Sunni insurgency, rates of violence against them would decline. Less predictable is how peaceful or not the incorporation of the "Sons of Iraq" (SOI) into the new Iraqi state will be.
This month saw the scheduled handover of the running and payment of more than 90,000 Sunni gunmen to the Iraqi government. US troops, who during the sectarian violence of 2005-07 provided a break against violence, are scheduled to be out of Iraqi cities by the end of June. Yesterday morning seven car bombs directed against Shia civilian targets killed 36 and injured more than 130 in what the US military described as a "coordinated attack by terrorists against predominantly Shia targets that they gauge as vulnerable to instigate sectarian violence".
It followed outbreaks of violence in Baghdad last week when Iraqi government forces detained Adel Mashadani, the fiery leader of the SOI group in Baghdad's Fadhil neighbourhood, followed by the arrest of another SOI leader, the more moderate and previous US ally, Raad Ali.
The present tipping point concerns the disarmament of the Sunni militias and their acceptance of the sovereignty of the Iraqi (Shia-dominated) government. Adam Silverman was part of the human terrain teams that saw a cadre of social scientists embedded within the military fighting the counter-insurgency. He interviewed a Sunni leader of one of the SOI groups and asked him what his response would be to Iraqi security forces clamping down on his militia. The man told Silverman that he would desert his post, come home, return to being a Son of Iraq, and resist the Iraqi Security Forces with his brother and the rest of the Sons of Iraq in the area.
With the potential for slippage obvious, disarming militias is delicate work indeed. Groups such as Hezbollah in Lebanon are testimony to the durability of sub-state groups within weak national states. The current attempt at the integration of forces into the state is a critical test of political trust as well as the Iraqi government's ability to place national above sectarian interests. Disarming the Sunni groups while allowing the peshmerga (beyond the fluid borders of the KRG) and Shia Badr brigades and the Mahdi army to remain armed is a difficult task.
With little real progress on national reconciliation some observers feel that the Iraqi civil war has yet to be fought, with all sides awaiting the imminent American departure. If the Sunni militias hand in their weapons it leaves them vulnerable to the sectarian violence of the type that dominated in 2005/2006. The patience of the SOI has already been strained since control transferred to the Iraqi government with many complaining about a drop in their wages, continued delays of their payment and regular arrests of members promised amnesty.
The "patience of peace" is a finite commodity and whoever targeted the market crowds and labourers yesterday aims to restart the cycle of killing that the surge tactics had significantly slowed.
This comes against a backdrop of the Americans being accused of taking their eyes off the ball in Iraq. While the Brits have packed up and been given a golden fish on their way out, Obama has made Afghanistan his war. The Nato summit placed much emphasis on increasing troop numbers there while the US has not even had an ambassador in Baghdad since Ryan Crocker left on 13 February.
American officials described the arrest of the SOI leaders as an Iraqi "internal matter". US Major Kone Faulkner said Iraq is "a sovereign nation with an elected government and capable criminal justice system designed to protect Iraqi citizens through the rule of law". Yet reports from the past week tell of multiple bombings, the assassinations of gay people and the trafficking of children.
The present incorporation of the Sunni insurgency into a more peaceful Iraq is a critical test for the Iraqi government. If last week's fighting in Baghdad is a precursor of a more violent stand-off to come, then the US may find its smooth exit from the country rudely interrupted.