Drop a pebble into a pond and, after the initial splash, ripples spread across the water. It's a familiar image, one we know so intuitively that everyone understands immediately what you mean by the phrase “ripple effect.” Yet, even when the pebble is a boulder, and the splash soaks us, we may not fully grasp how far the ripples will spread—or for how long. On March 20, 2003, the United States invaded Iraq. I crossed the berm two days later as part of the 101st Airborne Division (Air Assa...