Today's transportation infrastructure makes international trade more efficient than intra-U.S. trade was 100 years ago. What this means is that it is easier today for a shirt made in China to get to California than it was for a shirt made in Georgia to make it to Missouri in 1900. The U.S. Constitution has always banned states from regulating trade between states. This amounted to a within United States free-trade agreement. Can we use theexperienceof the United States between 1900 and 2000 to predict what would happen in world trade if there was free trade across the globe?