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.

That's the conclusion of a study released in June from the U.S. Army's Strategic Studies Group. Researchers from the group's Megacities Concept Team spent a year visiting the world's megacities — defined as having a population greater than 10 million people — to study how the Army might conceivably fight a war inside them.

"The Army, and the [Department of Defense] community more broadly, neither understands or prepares for these environments," the report states.