If you want to picture the characteristically 21st century living environment, imagine cities like Lagos and Shanghai; or Sao Paulo, New York and Cairo. Now double the population, and you have a pretty close approximation of what the world’s largest urban areas will be like by mid-century.
Now ask: How does the Army control a city of that size? How does an infantry force of thousands cut off and surround and city of 10 million? What about 20 or 30 million? The answer is that the United States can’t, at least not yet.