The killing of 16 civilians in Afghanistan, allegedly by a U.S. soldier, has further strained the relationship between the two countries. Plus, Rick Santorum won big in the South, yet Mitt Romney scored more delegates. And President Obama's re-election campaign is in full swing. Joining Gwen: Martha Raddatz, ABC News; Alexis Simendinger, RealClearPolitics; Karen Tumulty, The Washington Post.