From Forbes -

"Sadr City has been targeted by a large number of explosions that killed hundreds of people in the past months, including Nov. 23 bombings in which 215 people died. The Mahdi Army had been almost in full charge of security in the area, checking cars and suspicious people, and some have blamed the drop in their presence for car bombings that have continued despite the crackdown."

And now the people of Sadr City have to face the additional threat of random searches and detention by U.S. and Iraqi troops.

If you were a Sadrist, who would you want to protect you - The Mahdi Army who shares the same language and religion as you or the U.S. army who doesn't speak your language or share your religion?

Duh!