"From the Halls of Montezuma, To the shores of Tripoli, We fight our country's battles, In the air, on land, and sea." So opens the official hymn of the United States Marine Corps, and it covers the great scope of the American military. The Marines in particular find themselves fighting in all domains, with the Air Force, Army, and Navy each getting their fair share of air, land, and see. Yet as war evolves, and as the military looks to the future of combat, there are places it hasn't yet fought. The Pentagon wants ideas on how to fight a war in megacities and caves. And they want to know by next week.

Megacities are a relatively new phenomenon. When the Marines stormed the Halls of Montezuma in 1847, the entire population of the United States was just over 20 million. Today, 20 million people live in or around Mexico City itself. Underground transit systems are also pretty new. While Union and Confederate troops were fighting at the Battle of Arkansas Post, London's Metropolitan Railway began operation for the first time.