Free trade is again under attack, despite having been, for over a century, the basis of America's wealth. Some groups in the United States blame free trade for the loss of manufacturing jobs, while others blame it for exposing some U.S. producers to foreign competition. Free trade, however, is good for America, and for a very simple reason: It allows American workers to specialize in goods and services that they produce more efficiently than the rest of the world and then to exchange them for goods and services that other countries produce at higher quality and lower cost. Specialization and...