Ms. Priest talked about her book The Mission: Waging War and Keeping Peace with America's Military, published by W.W. Norton and Company. She argues that, traditionally reluctant to engage in humanitarian and peacekeeping assignments, the U.S. military, by virtue of taking them on in the 1990s, has nevertheless become the dominant day-to-day influence in foreign policy. Ms. Priest spent time in the field with top military brass and foot soldiers alike in such hot spots as Colombia, Nigeria, Afghanistan, Indonesia, and the Balkans, where she examined how operations are carried out and what those in the military think of their expanded roles, for which they are ill-trained.