It's not a huge deal, but it will complicate things for a more simple-minded crowd:
A secret deal between Britain and the notorious al-Mahdi militia prevented British Forces from coming to the aid of their US and Iraqi allies for nearly a week during the battle for Basra this year, The Times has learnt.
Four thousand British troops – including elements of the SAS and an entire mechanised brigade – watched from the sidelines for six days because of an “accommodation” with the Iranian-backed group, according to American and Iraqi officers who took part in the assault.
US Marines and soldiers had to be rushed in to fill the void, fighting bitter street battles and facing mortar fire, rockets and roadside bombs with their Iraqi counterparts.
There have, no doubt, been many "secret deals" between various factions in Iraq. You cannot blame the British for setting up a deal with the Mahdi Army--they had to find a way to operate in Basra. What you can point out is that the US did the exact same thing with Sunni elements. Both deals reduced violence against British and US troops. Both could be interpreted to mean that we're all just kicking the can down the road until we can all leave.
Britain's accomodation--and the revelation that part of the deal was the release of detained individuals--came about because of the siege that took place at the fortified palace in Basra.
Britain’s main base at an airport just outside the city was also under constant rocket and artillery fire.
In what turned out to be a misinterpretation of the violence as criminal-led rather than a full-blown insurgency, Britain cut a deal with the Sadrist militia to ensure the safe withdrawal of British troops from the Basra palace to the airport in return for the release of a number of militia prisoners, in what became known as the “accommodation”.
What's not clear is just what the reaction among the British and the Americans really was when it became evident that the Maliki government was going to freelance it and start a fight in Basra. It was either a debacle or a stunning victory. Troops mutinied but the Mahdi Army melted away. Did they demand that no future operations kick off like that again? Or did another secret deal ensure everything would settle down until an eventual British withdrawal?
Or did it convince the British that their imminent withdrawal was long overdue?