Now, as Moqtada al-Sadr is playing the "bad cop" - or "bad cleric" in
this case - al-Sistani - the "good cleric" - with his followers probably
will extract from the occupiers a heavy price for their peacefulness.
After all, the occupiers (should) know, that if al-Sadr´s rather feeble
militia can cause to them these kind of troubles, the bigger, better
trained other Shia militas with far larger support are able to create
far greater havoc and casualties - and most likely, wrench the country
from their hands (Iraqi Kurdistan excepted).

Of course, Americans are rather blind to the realities of life and war.
If the use of force don´t bring to them what they want, then they
think that the use of greater force will. Think about Richard Nixon:
He was withdrawing American forces from Vietnam for much of his
presidency, but he was also increasing the number of bombings
(and Vietnamese casualties) until 1973. He too thought that force
will work. It didn´t. It rarely does, if used alone.