The fighting this week in Basra may be a prelude to the moratorium's collapse and, with it, the resumption of wide-scale sectarian violence—Shiite vs. Sunni and Shiite vs. Shiite.
Many Shiites believe—not unreasonably—that Maliki ordered the offensive in Basra now in order to destroy Sadr's base of support and thus keep his party from beating ISCI in the upcoming provincial elections.
Late last month, Iraq's three-man presidential council vetoed a bill calling for provincial elections, in large part because ISCI's leaders feared that Sadr's party would win in Basra. The Bush administration, which has (correctly) regarded provincial elections as key to Iraqi reconciliation, pressured Maliki to reverse his stance and let the bill go through. He did—at which point (was this just a coincidence?) planning began for the offensive that's raging now.
Also this suggestion of why the US supports ISCI--mysterious because it has stronger ties with Iran--would not surprise me if it is actually true:
The current fighting in Basra is a struggle for power and resources between those warlords. It's hard to say which faction is more alluring or less likely to fall under Iranian sway. Neither seems the sort of ally in freedom and democracy that our president conjures in his daydreams. (The lively blogger who calls himself Abu Muqawama speculates that Bush officials have embraced ISCI because, unlike Sadr, its leaders speak English.)