In 1939, two years before the Japanese bombing of Pearl Harbor drew the United States into World War II, the U.S. military was an anemic force in which Army troops still used horses to pull around artillery. Then an emergency buildup ordered by President Franklin D. Roosevelt transformed U.S. automakers’ manufacturing plants into extraordinarily efficient producers of tanks, guns, airplane engines and more.One Ford plant in Ypsilanti, Mich., was able to build an entire B-24 Liberator long-range bomber — with 1.55 million parts — in just over an hour. As much as anything, FDR’s directive helped…