Brick by brick, President Donald Trump is building a wall around the world’s largest economy. As America’s tariff barriers on everyone else have gone up, so has the drawbridge, making it harder for migrants to enter the country. The president wants to turn America into a fortress that keeps out foreign incursions. In fact, he is cutting America off from the very goods and talent that helped make its economy the envy of the world. Already the damage is starting to show; once wreaked, it will not easily be reversed. That is not how investors see it. In the six...