Three times President Obama has been called upon to deliver words of grace and healing after an appalling mass shooting: after 6 were killed in Tucson, Arizona, after 26 (including 20 children) died at Sandy Hook Elementary School in Newtown, Connecticut, and most recently after nine were gunned down in a prayer group in Charleston, South Carolina. Each time, President Obama has been up to the task, and has delivered stirring words that have moved...