For months, calming statements from the ayatollah held Shiites back from retaliating for killings by Sunni insurgents. But three years of insurgency, sectarian tensions and miserable living conditions have shrunk the space for temperance and given extremists plenty of room to operate. "[Sistani] doesn't have the same degree of influence," says Joost Hilterman, director of the International Crisis Group's Iraq program, based in Jordan. "He may be saying the same things, but fewer people are listening to him." As much as anything, the battle now is about which voices will shape the future of Iraq. Not too long ago Sistani...