The CEO of a prominent U.S. aluminum producer recently argued on CNBC that tariffs on foreign aluminum were good policy because aluminum is vital for national defense.He noted the many different ways in which the military relies upon varieties of aluminum and said that the U.S. could not risk having its supply cut off in times of trouble.Is this a valid argument?If aluminum is vital to national defense, then what about American-made automobiles?American-raised and -produced sugar?