It cannot be said often enough that the chief of staff of the United States Army, Gen. George Casey, responded to a massacre of 13 Americans in which the suspect is a Muslim by saying: "Our diversity ... is a strength."As long as the general has brought it up: Never in recorded history has diversity been anything but a problem. Look at Ireland with its Protestant and Catholic populations, Canada with its French and English populations, Israel with its Jewish and Palestinian populations.Or consider the warring factions in India, Sri Lanka, China, Iraq, Czechoslovakia (until it happily split up), the Balkans and Chechnya. Also look at the festering hotbeds of tribal warfare — I mean the beautiful mosaics — in Third World hellholes like Afghanistan, Rwanda and South Central, L.A."Diversity" is a difficulty to be overcome, not an advantage to be sought. True, America does a better job than most at accommodating a diverse population. We also do a better job at curing cancer and containing pollution. But no one goes around mindlessly exclaiming: "Cancer is a strength!" "Pollution is our greatest asset!"