Garton Ash believes that "capacity to intervene" will diminish as Basra increasingly comes under the sway of extremist elements. AlterNet concurs: "Should British forces decide to venture back, they will inevitably face a den of Mahdi Army fighters," according to their analysis: The militia is said to number 17,000 in Basra alone and is divided into 40 company-size military units, according to a senior Iraqi security official. ... They control multiple units in the 14,500-strong police force, and hold sway in hospitals, the education board, the university, ports and oil terminals, and the oil products and electricity distribution companies.

“We teach each other,” i.e. cross-pollination. The traffic’s not one way, either; follow that last link to refresh your memory. As far as I know, it still hasn’t been conclusively established that Sadr is Iran’s chief proxy in Iraq, although the collusion with Hezbollah and the fact (again, per the Independent) that thousands of JAM members have trained in camps in Iran indicates a high level of nuance to the relationship. Traditionally SCIRI was Iran’s arm inside the country but they seem to have drifted towards the American side in the past year, possibly as a reaction to Sadr’s coziness with the regime. Michael Totten, on the ground in Baghdad, seems pretty clear on what’s what, though: The Mahdi Army is Iran’s major proxy in Iraq. It is, in effect, the Iraqi branch of Hezbollah.