One of Ours

...the situation in Najaf is different from that in Fallujah because the Shiites are themselves divided, whereas in Fallujah the Sunnis were relatively united.
[...]
The reason the disunity matters is that it allows the US to be more successful in its siege of Najaf than it was in Fallujah....Because Grand Ayatollah Sistani and Abdul Aziz al-Hakim are actively colluding with the Americans to destroy Sadr and his militia, the US can hope to succeed by force, without having to do the hard work of making a political deal with the Sadrists.

Muqtada himself is largely responsible for his lack of significant Shiite allies, since he has deliberately alienated them all by bullying and threatening them...