During Friday’s 9/11 commemorations, it was startling to realize that one-third of the American population today wasn’t born when it happened. To those of us of an older vintage, Sept. 11, 2001, seems like it was yesterday. The terrorist attacks at the hands of Osama bin Laden and al-Qaida, which killed nearly 3,000 people, leveled the World Trade Center and damaged the Pentagon, marked a turning point in our history. Whatever sense of invulnerability America may have had as the world’s only superpower was shattered, and it precipitated massive changes in the way we lived, traveled and perceived ourselves. We...