Hour 2: What events precipitated the U.S.-Mexican War and how did the conflict influence our young country? We’ll look back this hour with Amy Greenberg, Edwin Erle Sparks Professor of History and Women’s Studies at Penn State University and author of the new book “A Wicked War: Polk, Clay, Lincoln, and the 1846 U.S. Invasion of Mexico” (Knopf, 2012).