Sunday wasn't just another in a long series of tragic days in Iraq. When Shi'ite militias loyal to the cleric Muqtada al-Sadr launched a series of coordinated attacks yesterday, it marked the beginning of what could be the most dangerous phase yet of the allied occupation.

"The difference here is the type of violence and who's fighting," Josh Marshall explains.

Almost all the paramilitary violence -- the shootings and ambushes and roadside explosions -- have come from the Sunni minority concentrated in the center of the country.

Violence from the Sunni areas has never been difficult to understand. They lost out on privileges and status when Saddam was overthrown. And the future looked even more bleak, because the eventual handover of authority to a democratically elected government all but insured the domination of the Shia majority which the Sunnis had been lording it over for decades if not centuries.

In other words, time was never on the side of the Sunni rejectionists. From the start their interests were in destabilizing and delegitimizing the occupation.

Time, however, was very much on the side of the Shia. From a cynical viewpoint, why not let their American and Sunni enemies bloody each other into exhaustion in the central Iraq and sit back and wait on the day -- not too distant, certainly -- when they would inherit the new Iraqi state?

A central question has always been, when would the Shia come off the sidelines? ... Now, they seem to have come off the sidelines with a vengeance.

Juan Cole says that by closing down Sadr's newspaper, and going after one of his top capos, the Coalition Provisional Authority baited the cleric into this fight. But why?

THERE'S MORE: The counter-attack against Sadr has begun, Cole and Chris Allbritton report. He's locked himself inside a mosque, David Koresh-style. Will U.S. forces do him the favor of turning him into a martyr?

Meanwhile, 2,500 Marines have encircled Falluja, to retaliate for last week's heinous attacks there.