THE UNITED STATES is not “full.” In fact, it is empty. Right now, the country has about 93 people per square mile. Many, many countries are far denser than this, and not just city-states like Singapore (more than 20,000 per square mile) or small island nations like Malta (3,913 per square mile). South Korea has 1,337 people per square mile, and Belgium has 976. If you tripled the population of the United States, adding the new Americans only to the Lower 48 and leaving Alaska and Hawaii intact and unchanged, the main part of America would be only about as dense as France and less than half as dense as Germany.
A transformation on that scale is almost impossible to imagine, in large part because the American political system has…