Aboard the Navy’s newest aircraft carrier in early March, President Trump vowed that the United States “will have the finest equipment in the world—planes, ships and everything else.” But what good will this equipment do if the military lacks the personnel to use it?

People are the vital ingredient to America’s military edge, but increasingly they are in short supply. “The Air Force has a shortfall of almost 1,500 pilots,” Joint Chiefs Chairman Gen. Joseph Dunfordtestified before a House committee in March. Similarly, the...