Clinton-Obama Campaigns; Sunni vs. Shia; Andrew Roberts

Journalists Adam Nagourney of The New York Times and Mark Halperin of ABC News discuss the open conflict arising between the Clinton and Obama campaigns. Middle East experts Vali Nasr, Bobby Ghosh, Richard Bulliet, and Raghida Dergham examine the split between Shia and Sunni Muslims, which has once again given rise to sectarian violence in Iraq. Historian and journalist Andrew Roberts shares his new book, "A History of the English-Speaking Peoples Since 1900," and offers his thoughts on Tony Blair.