Iraqi forces clashed with Shiite militiamen Tuesday in the southern oil port of Basra and gunmen patrolled several Baghdad neighborhoods as followers of Shiite cleric Muqtada al-Sadr ordered a nationwide civil disobedience campaign to demand an end to the crackdown on their movement.

The potential impact is huge and this could be the beginning of the end of the decrease in violence that we've seen over the past few months.

No knows for...