A debate is raging these days over the future of American national security strategy, and the size and shape of the military force needed to implement it. Nearly everyone agrees that the United States needs a less expensive approach and a smaller military. But what should be cut—and by how much?

During past military drawdowns, cuts were proportional across the Navy, Air Force, Army, and Marines. This was not just to avoid inter-service fights; it also reflected the idea that there was the right balance among the military branches. But the current round of cuts might be different. A growing number of defense experts, retired military officers, and former defense officials contend that the U.S. military should be optimized for high-tech, precision attacks from long distances. When boots on the ground are absolutely necessary, the preference is for quick strikes by Special Operations Forces rather than prolonged involvement by battalions of soldiers and Marines. The use of landpower, the argument goes, would repeat the past decade's involvement in Iraq and Afghanistan—and thus, the U.S. should downplay its use, shrinking ground forces more than the Air Force or Navy.

Admiral Gary Roughead, a former chief of naval operations, and defense expert Kori Schake, argue as much in a recent Brookings study. Equating landpower with "manpower-intensive, sustained ground combat or counterinsurgency operations," Roughead and Schake recommend deep cuts in the Army and moving much of its remaining capability from active to reserve while keeping the Air Force and Navy at roughly their current size. This argument is not unique or even unusual, but emblematic of a school that portrays landpower as old fashioned and high-tech air and seapower as forward leaning. The appeal is understandable, as such a strategy would harness America's technological edge and offers the promise of less costly wars. But beneath the promise of using advanced technology to limit casualties and undertake clean, quick wars is a dangerous degree of strategic nostalgia. We have been here before. The argument is not new—just new people are making it.

Relying on a high-tech military optimized for standoff strikes makes sense only under certain conditions. It requires readily identifiable targets important enough to attack with a high-tech, scarce, and expensive weapon—and enemies inclined to accept defeat if a certain proportion of those targets are destroyed by American missiles or bombs (or at least be deterred by the possibility of having those targets destroyed). In other words, if the only opponents that matter are other nations—whether Iran, North Korea, or, possibly, China—and if all the U.S. wants is to defeat identifiable enemy militaries and hope someone else builds a sustainable peace that prevents the conflict from spreading or recurring, then an American military focused on high-tech, long-distance strikes makes sense.

Unfortunately, this scenario doesn't reflect the current reality, and isn't likely to materialize in the future. The emerging world is not like the 19th or 20th centuries, where all the U.S. had to do to remain secure was prevent a hostile great power from directly ruling Eurasia or interfering with the sea lanes. The mass connectivity and onrushing urbanization of the 21st century are spawning new types of threats very different from ponderous state militaries with identifiable centers of gravity. The enemies of the future will blend the somewhat conventional use of advanced weapons with decentralized, swarming networks relying on raids, sabotage, terrorism, and "area denial" weapons such as landmines and IEDs.

As futurist Martin Libicki noted more than 15 years ago, the 21st century security environment will be dominated by the "small and the many" rather than the "few and the large." A hybrid, networked opponent operating in a densely populated urban area cannot be defeated by destroying a limited number of targets. Its defeat requires a deep and constantly evolving understanding of both the enemy network and its cultural, social, political, and economic environment. Only a resilient, versatile and adaptable U.S. military with direct knowledge of the situation can succeed against this type of enemy.

Time is of increasing importance to military response. With the speed of communication now, security threats and modes of armed conflict evolve rapidly. The advantage goes to the fastest, most adaptive and versatile militaries. As strategic theorist Edward Luttwak wrote, the "paradoxical logic of strategy" has always meant that what works today often will not work tomorrow as the enemy adjusts. The more a military is based on large, expensive, complex platforms rather than human teams, the less adaptable and versatile it will be.

It would be nice if the United States could simply opt out of all messy conflicts, but it cannot. Global connectivity means that conflict in any part of the world has cascading effects. These are most intense in neighboring states or regions as combatants, refugees, money, disorder, crime, and weapons flow back and forth, but in most cases will spread even further. The recent conflict in Libya shows this contagion effect, when there is no sustainable security following the defeat of an enemy regime. In the future, major conflicts anywhere will affect the global and American economies, increasing commodity prices, disrupting the supply of goods and services, and creating uncertainty. U.S. economic growth will depend, in part, on whether the global economy is generally stable or conflict-ridden. This will make it difficult or impossible for the United States to totally avoid major conflicts (although it does not mean the U.S. will intervene militarily in every major conflict). The profusion of global diasporas will also make it politically difficult to ignore major crises or conflicts.

However much the U.S. wishes for high-tech combat against a less-high-tech foreign military, to assume that future conflict will be like that is dangerously nostalgic. Since World War II, the U.S. almost never used its military where it expected. The war in Europe never materialized, while unanticipated ones in Korea, Vietnam, Kuwait, and Afghanistan did.  Other than the 1991 war against Iraq, the American military seldom fought the type of enemy that it had expected: conventional forces relying on Soviet equipment and tactics.

It would make things much easier if the U.S. could optimize its armed forces to defeat a convenient enemy like an identifiable foreign military. But in all likelihood, the U.S. military will face enemies it didn't anticipate in places it didn't expect. To hope that future enemies will all be technologically inferior conventional armed forces is more nostalgia than a hard-headed assessment of the future of armed conflict. Success will come from adapting faster and more effectively using direct involvement with a range of human teams and organizations, and from active involvement in building security before conflict erupts.

As Americans think about the type of military they want, they should not be beguiled by promises of quick, low-casualty conflict based on big, extremely expensive, high-tech weapons. Instead, they must look at the situation today and pay for a military that best reflects it, rather than one designed for the kind of war that is slowly fading away.