Other conflict environments differ widely from theater to theater. U.S. forces at any given time are deployed on or under water, in littorals, jungles and deserts and in 10,000-foot mountains, posing other problems.

The combat environment is just as diverse and complicated. The panoply of force protection—from body armor to protected vehicles and counter-rocket/artillery/mortar defenses—did not exist a decade ago. Strategist Andrew Krepinevich likes to talk about ”cost imposition strategies”: The use of improvised explosive devices is one such, employed very successfully against the developed world.

For military planners and their governments, there is another environment to be considered at all times—that of economics. In the U.S., the enormous cost of sustained boots-on-the-ground warfare, together with political unwillingness to raise taxes, has been a factor in driving national debt to crisis levels likely to force major changes in the armed forces and defense industry.

A cash-strapped United States is looking to allies to bear a higher share of what it sees as the common defense—the U.S spends 4.8% of its gross domestic product on its military while the best of its European allies struggle to make a 2% target. But those allies are trying to restrain their own domestic spending and their governments are under pressure to preserve social spending.

Demographic stresses complicate the economic trends. Aging populations require more maintenance, and there are relatively fewer young people, either to support them or join the armed forces. In the U.S., a decade of large-scale combat deployments has forced the services to pay more: Krepinevich calls the 50% increase in personnel costs “eating the budget from the inside out.”

The problem is to allocate resources to meet these many challenges. This involves accepting that there is not enough money to deal with them all as one would wish, and that means accepting risk in some areas. Picking those areas is what strategy is about.