We must remember not just South Vietnam's fall, but why it fell.Fifty years ago, April 30, 1975, the world watched in horror and disbelief as the last American helicopter lifted off from the rooftop of our embassy in Saigon. South Vietnam had fallen in the manner of Ernest Hemingway, “first gradually, then suddenly”: a decades-long war, a relative peace, and then a mad dash by the North Vietnamese Army that consumed the country in less than a month. The tragedy was simply breathtaking. And horribly, horribly unnecessary.What followed was not peace, but darkness. The swift collapse of South Vietnam, Cambodia,...