It's Badr Versus Mahdi in Karbala

On one side of the riots in the Shiite-dominated city of Karbala is the Badr Brigade — affiliated with the Supreme Islamic Iraqi Council. On the other is the Mahdi Army — affiliated with cleric Muqtada al-Sadr.

According to scholar Vali Nasr, both groups have had close ties to Iran in the past, but now the Badr army is becoming closer to the United States and the Iraqi military.

Nasr, author of "The Shia Revival: How Conflicts within Islam Will Shape the Future" and a professor of International Politics at Tufts University, talks with Robert Siegel.