Keeping up: training near Fort Eustis, Virginia, July 2008 (Robyn Gerstenslager / U.S. Navy)

Over the next decade, the U.S. military will need to undertake the most dramatic shift in its strategy since the introduction of nuclear weapons more than 60 years ago. Just as defense budgets are declining, the price of projecting and sustaining military power is increasing and the range of interests requiring protection is expanding. This means that tough strategic choices will finally have to be made, not just talked about. As the British physicist Ernest Rutherford once declared to his colleagues, "We haven't got the money, so we've got to think."

A new strategic framework will be needed, one focused less on repelling traditional cross-border invasions, effecting regime change, and conducting large-scale stability operations and more on preserving access to key regions and the global commons, which are essential to U.S. security and prosperity. The bad news is that this will mean reducing the priority of certain objectives and accepting greater risk in some realms. But the good news is that with a shift in focus, truly critical U.S. interests can continue to be protected at a sustainable cost.

DECLINING RESOURCES

After the Cold War, Washington enjoyed a "unipolar moment," drawing on its overwhelming advantage in resources and technology to achieve an unprecedented level of global military dominance. Two decades on, however, that moment is fading. The U.S. economic engine is sputtering, with unpleasant implications for a Defense Department that has grown accustomed to steady budget growth.

From 1999 to 2011, annual U.S. defense spending increased from $360 billion to $537 billion in constant dollars, not including an additional cumulative $1.3 trillion spent on operations in Afghanistan and Iraq. The Obama administration and Congress have already agreed to pare back planned further increases over the next decade by a total of nearly $487 billion. In January 2013, the budgetary process known as "sequestration" is set to trigger another $472 billion in total reductions over the same period. Congress may avoid sequestration by finding other ways to lower the federal deficit, but even if it does, additional major cuts in defense spending are likely to come eventually. And if history is any guide, most of the $200 billion in "efficiency" savings over the next five years that the Pentagon is currently counting on will fail to materialize.

This means that serious belt-tightening is coming -- a process that will be made even more difficult thanks to increasing manpower costs and the decline of the country's European allies. Since its inception in the 1970s, the United States' all-volunteer military has been a source of strength, generating a highly professional force. But the recent wars in Afghanistan and Iraq have revealed the volunteer force's Achilles' heel: in order to attract large numbers of qualified personnel willing to serve in dangerous and unpleasant wartime conditions, the Defense Department has had to raise salaries and benefits substantially. Even adjusted for inflation, total military compensation has increased by nearly 50 percent over the past decade -- an unsustainable rate of growth.

Fielding a volunteer force also means keeping casualties low or wars short. To protect its troops from the enemy's use of cheap roadside bombs in Afghanistan and Iraq, the Pentagon spent over $40 billion on thousands of new heavily armored vehicles, along with over $20 billion to better detect the bombs.

In the past, the United States might have looked to its wealthy and technically advanced allies, such as France, Germany, and the United Kingdom, to step up and fill the gap. Yet while all these countries have long and impressive martial traditions, their current militaries and defense industries are faint shadows of their former selves. Each spends on defense less than half of what the United States does as a percentage of their GDPs, and in real dollars, they spend only one-quarter as much combined. They can help preserve a stable international system, but they act largely as free riders on the United States for their security. The United States' Pacific allies, such as Japan and Australia, might be willing to shoulder a greater burden in their region, but they have yet to augment their defenses enough to make a significant difference.

GROWING CHALLENGES

As the United States and its allies reduce their military spending, the world is experiencing growing turbulence. For decades, the United States has sought to prevent hostile powers from dominating critical regions, such as western Europe, the western Pacific, and the Persian Gulf, while preserving unfettered access to the global commons (the seas, space, and now cyberspace). Following the Cold War, the threat to Europe's security declined dramatically, but the same cannot be said about the other two regions or the commons, since rivals such as China and Iran are working to shift regional military balances in their favor.

Perhaps the most striking development is the progressive loss of the U.S. military's near monopoly on precision-guided munitions, or "smart bombs." China, in particular, is moving to exploit the effectiveness of precision-guided munitions to achieve its strategic goals. The People's Liberation Army (PLA) is incorporating precision guidance into its ballistic and cruise missiles and strike aircraft to enhance their ability to strike fixed targets with high accuracy over long distances. This has profound implications for the U.S. military's traditional way of projecting power, which relies heavily on deploying and supplying forces through major ports, logistics depots, and air bases. The PLA is also looking beyond the ability to conduct precision strikes against fixed targets and is developing missile systems designed to strike mobile targets, such as the U.S. Navy's aircraft carriers. And the PLA is aggressively pursuing both antisatellite and cyberwarfare capabilities to target the U.S. military's information and communications systems. Together, these "anti-access/area-denial" (A2/AD) capabilities will significantly increase the risks to U.S. forces operating in the western Pacific.

With a similar goal of regional hegemony but fewer resources, Iran is pursuing more modest A2/AD capabilities, including antiship cruise missiles, sophisticated antiship mines, and submarines. Iran is looking to combine these with large numbers of small boats capable of conducting "swarm attacks" on U.S. warships and with growing numbers of ballistic missiles that can range far beyond the Gulf. Tehran appears to have several objectives in mind, including transforming the Persian Gulf into a "no-go zone" for the U.S. Navy and, by extension, eroding the confidence of the United States' regional partners in Washington's reliability. Iran might also eventually provide precision-guided weaponry to clients and proxies such as Hezbollah and other local militant groups, helping them present a more lethal threat to U.S. expeditionary forces. Finally, Iran appears intent on acquiring nuclear weapons.

The secure access to the commons that the international community has come to take for granted, meanwhile, is being increasingly challenged. U.S. control of the open oceans is not in question, but within this decade, the proliferation of submarines, long-range antiship cruise missiles, and "smart" antiship mines could make transiting maritime chokepoints, such as the Strait of Hormuz, a hazardous proposition. Space-based satellites -- essential components of both the global economy and U.S. military effectiveness -- are becoming increasingly vulnerable to the PLA's antisatellite lasers and missiles. And the critical infrastructure behind everything from the United States' power grid and energy pipelines to its financial systems and e-commerce is either poorly defended against cyberattacks or not defended at all.

In the past, freedom of the seas has mostly meant freedom of traffic over them. In recent decades, however, a vast undersea economic infrastructure has emerged, located primarily on the world's continental shelves. It provides a substantial portion of the world's oil and natural gas, and it hosts a web of cables linking much of the Defense Department's Global Information Grid. Capital assets on the U.S. offshore seabed alone are estimated to exceed $1 trillion. And major new oil and gas finds in the eastern Mediterranean, along with prospective discoveries in the South China Sea, guarantee that undersea infrastructure will grow further.

Yet undersea capital assets are effectively undefended, since, as with the Internet, their developers assumed a benign geopolitical environment. Until recently, this did not pose a serious problem, as these assets were generally inaccessible. Now, however, technological advances are rendering critical undersea infrastructure increasingly vulnerable to potential enemies. Once the possession of only the most advanced navies, autonomous underwater vehicles, or robotic submersibles, are now commercially available and capable of carrying explosives and other contraband cargo. Latin American drug runners employ submersible and semisubmersible craft to move their freight, and it appears that other nonstate entities will be increasingly able to operate in the undersea domain.

REDUCING THE GAP

At its core, strategy should provide a guide for using available resources to achieve realistic objectives. Because the resources available to the U.S. military will be increasingly limited, the objectives must be, too -- lest the result be a hollow strategy that neither worries enemies nor assures friends. This means setting priorities among competing security objectives. The strategy of assured access summarized here would do this by reordering goals, selectively taking on risk, and exploiting the United States' strengths and its rivals' weaknesses.

Since the end of the Cold War, Washington has placed a high priority on maintaining the ability to wage two major wars simultaneously in Northeast Asia and the Persian Gulf. The focus has been on defending key allies and partners from traditional cross-border ground invasions (such as North Korea's attack on South Korea in 1950 and Iraq's attack on Kuwait in 1990) and defeating the aggressors, if need be, through counterinvasions aimed at regime change.

But today's threats are different. Neither China nor Iran is emphasizing an updated version of Soviet tank armies or Iraq's Republican Guard. The current and future challenge to stability in the western Pacific and the Persian Gulf is not a cross-border invasion but the spread of A2/AD capabilities, which will make it increasingly difficult for the United States to operate freely in those areas.

In recent years, the U.S. military has undertaken a number of operations aimed at regime change, most notably in Afghanistan and Iraq. These have successfully deposed hostile governments but have required protracted large-scale stability operations afterward that have been both costly and indecisive. The American public appears to have little stomach for any more such operations, except perhaps in response to a major attack against vital U.S. interests. Occupying hostile territory, moreover, is likely to become even more difficult over time thanks to the diffusion of what the Pentagon calls "G-RAMM" -- guided rockets, artillery, mortars, and missiles. And if the prospect of conducting a regime-change operation against a country the size of Iran is daunting, in the case of China, it is pure moonshine.

Fortunately, the United States has no need to set such ambitious objectives, since what it really wants is not conquest but access. The challenges that China and Iran pose for U.S. security lie not in the threat of traditional cross-border invasions but in efforts to establish spheres of influence in, and ultimately to control access to, critically important regions. What the Pentagon should set its sights on, therefore, is not optimizing U.S. forces to be able to produce regime changes through counterinvasions but a return to the more modest objective of forward defense: deterring regional aggression or coercion and protecting the global commons from major disruption.

With this shift in focus, A2/AD capabilities -- which favor defense -- become not problems for the United States and its allies but tools to be used by them, since the onus of power projection would fall not on Washington but on its opponents. Toward this end, the United States should work with its allies and partners in the western Pacific and the Persian Gulf to create local air- and sea-denial networks that would make aggression difficult, costly, and unattractive. South Korea and Taiwan could be important contributors to such regional defense networks, but Japan will be the linchpin in any U.S. strategy to maintain stability and access in the western Pacific. Tokyo should increase its investment in A2/AD capabilities, such as submarines, antisubmarine-warfare aircraft, antiship cruise missiles, defensive mining, air and missile defenses, and military base hardening and dispersion, reducing both the likelihood of any Chinese or North Korean attack and the burden on U.S. forces responsible for the defense of Northeast Asia. And similar investments by the members of the Gulf Cooperation Council would help minimize any regional threat posed by Iran.

Efforts such as these would enable U.S. forces in both regions to emphasize their unique strengths, such as long-range strike systems that can operate beyond the range of rival A2/AD systems and nuclear-powered attack submarines that can operate effectively within enemy A2/AD barriers. Increasing the number of forward hardened air bases, meanwhile, would allow U.S. strike fighters to be dispersed in forward positions, reducing the risk of their being destroyed in preemptive missile attacks and thereby enhancing crisis stability.

Such a strategy would play to several of the United States' competitive advantages, among them its broad network of regional alliances and partnerships and its lead in many advanced military systems. It would also exploit regional geography. A U.S. defense architecture in the western Pacific based along the so-called first island chain (from the Kuril Islands, through Japan and the Ryukyu Islands, to Taiwan and the Philippines) would protect U.S. allies and partners there while hemming in hostile naval forces during a conflict. And Iran has no choice but to export its oil and gas through either the Strait of Hormuz or pipelines traversing foreign territory. The Gulf Cooperation Council states, in contrast, could expand their capacity for exporting energy resources via the Red and Arabian seas, providing them with options in the event of a blockage of the Strait of Hormuz and leaving Iran in a position of comparative economic vulnerability. Should Iran attempt to route its exports through ports along the Arabian Sea, the U.S. Navy would be well positioned to interdict its shipping.

This shift in focus would hardly mean granting opponents territorial sanctuary or immunity from attack. The United States has the ability to conduct long-range penetrating strikes against its foes -- by planes, missiles, and submarines -- and it needs to do whatever is required to maintain that. U.S. special operations forces have shown the ability to operate effectively in hostile territory, especially when supported by strike aircraft and drones. The U.S. military also appears to maintain an impressive arsenal of cyberweapons that can be employed with potentially devastating effects against an enemy's critical infrastructure and armed forces. And if necessary, U.S. naval forces can pressure enemy homelands through blockades.

Deterrence through denial is designed to convince a would-be aggressor that he cannot achieve his objective, so there is no point in trying. Deterrence through punishment is designed to persuade him that even though he may be able to achieve his objective, he will suffer so much as a result that his anticipated costs will outweigh his gains. Deterrence by denial should be possible in both the western Pacific and the Persian Gulf, because the coming environment will likely privilege defense over offense in conventional military operations. When it comes to protecting the commons, however, the vast array of soft potential targets -- undersea infrastructure, satellites, computer networks -- gives the advantage to offense, and so here the United States and its allies will need to rely more on deterrence by punishment.

This may be complicated by the difficulty of figuring out who is responsible for any attack -- a prerequisite for punishment. When it comes to undersea, space, and cyberspace attacks, therefore, the Pentagon should make it an urgent priority to enhance its post-attack forensics and other forms of intelligence aimed at identifying attackers. The quicker and better it can do this, the more able it will be to reestablish deterrence by punishment as a central element of assuring access to the commons. Even accurate attribution may not be sufficient to deter attacks should nonstate groups achieve a greater ability to conduct cyberwarfare and to operate under the seas, as they may not fear retaliation or may have few assets against which to retaliate. So the Pentagon will also need to plan on developing ways of limiting the damage from such attacks and figuring out how to rapidly reconstitute damaged or destroyed assets.

Despite the desire of many powers in the developed world to reduce their reliance on nuclear weapons or even eliminate them entirely, such weapons retain their popularity. Russia's military doctrine has increased the country's reliance on nuclear weapons to help offset its weak conventional forces. China retains a strong nuclear force. Pakistan is building additional nuclear reactors to produce fissile material for its expanding nuclear arsenal to offset India's advantage in conventional forces. Iran is driving to a nuclear capability, which could spur an additional proliferation cascade in the Middle East. In light of all this, the Defense Department needs to undertake a comprehensive nuclear posture review that addresses multiple plausible contingencies of nuclear use, coming up with practical policies for how such uses might be prevented or, failing that, how a nuclear conflict might be terminated on acceptable terms once begun.

No strategy is without its risks, and this one relies significantly on the actions and investments of regional allies and partners -- never a sure thing. It is encouraging, however, that many U.S. partners in the Asia-Pacific region have expressed growing concerns over China's military buildup and increasingly assertive territorial claims. These concerns must now be translated into investments in military capabilities. The United States' Arab partners in the Gulf, in contrast, are already making substantial investments in their defenses, but they need to focus their efforts on countering Iran's attempts to assert regional hegemony. The strategy also rests in part on the anticipation that countries sharing common interests with the United States, such as India and Indonesia, will welcome efforts to preserve the stability of, and free access to, the greater Indo-Pacific region.

A strategy of assured access reflects a sense of what the U.S. military can realistically achieve. Should deterrence fail, the objective of any military action would be to restore the status quo ante, rather than pursue broader Wilsonian ideals and a resolution of the supposed root causes of the problem. The goal would be to keep a conflict limited, even at the cost of forgoing definitive or optimum outcomes. This would be similar to the Iraq strategy of President George H. W. Bush, not that of President George W. Bush, the Korea strategy of President Harry Truman, not that of General Douglas MacArthur. It might forgo a hypothetical best option -- but letting such a best outcome be the enemy of an acceptable one would be a mistake of the first order, especially when the best is a dangerous illusion.

HARD CHOICES

Strategy is about setting priorities, and not everything can be a top priority. So if an assured-access strategy gives priority to maintaining access to critical regions and the global commons, what parts of the existing U.S. approach does it not include? What objectives does it leave out, and what greater risks does it accept in order to narrow the gap between strategic objectives and resource limitations?

Major defense economies, it suggests, could come from deeper reductions in U.S. ground forces. The U.S. Army and the Marine Corps, which were expanded to meet the demands of the wars in Afghanistan and Iraq, are already being reduced. But the currently planned drawdown will still leave them larger than they were on 9/11, when U.S. strategy called for them to wage two major regional wars, including regime-change operations. So there is room to cut further.

The United States could scale back, for example, the ground force contributions it plans on making in any major conflict on the Korean Peninsula. The threat from North Korea has changed radically since the early 1950s, from a traditional ground invasion to a massed artillery barrage including missiles and weapons of mass destruction. South Korea has twice the population of North Korea and boasts one of the world's largest economies. Its ground forces are both large and competent. The greatest comparative advantages the U.S. military has relative to its South Korean counterpart are in air and sea power. The time has come, accordingly, to acknowledge that Seoul is fully capable of assuming primary responsibility for its own ground defense, and it should do so.

When it comes to stability operations, the Pentagon should commit seriously to its already de facto strategy of using an "indirect approach" to preserving order in the developing world outside the western Pacific and the Persian Gulf. The U.S. military's comparative advantage in counterterrorist and stability operations lies in the quality of its manpower, not its quantity. U.S. forces are simply too expensive to be committed in large numbers to the defense of peripheral interests. This means avoiding direct U.S. interventions and instead emphasizing training, advising, equipping, and supporting allies and partners confronting internal security threats. In ungoverned areas, it means relying on such "light footprint" alternatives as robotic scout and strike aircraft, which special operations forces can use to form hunt-and-kill teams to suppress hostile groups.

In meeting U.S. security commitments to Europe, moreover, the objective should be to maintain NATO while minimizing costs. To that end, Washington should place greater emphasis on nuclear guarantees to meet its Article 5 security commitment and reevaluate its plans to deploy elaborate missile defenses with little or no monetary contribution from the European allies being defended.

As for its defense policy, the Pentagon can narrow the means-ends gap and save money by increasing its emphasis on recapitalizing equipment rather than modernizing it -- that is, by stressing in-kind replacement as opposed to fielding entirely new generations of weapons systems with their high development costs. Such systems should be fielded only when the technical risks associated with doing so are minimal and when senior leaders have high confidence that the new systems will provide a dramatic and enduring boost in military effectiveness.

Where possible, the United States should use its resources in ways that impose disproportionate costs on its rivals. An important source of the United States' competitive advantage in this area is its long and distinguished history of "black" programs, which have produced the atomic bomb, the U-2 and SR-71 spy planes, stealth aircraft, and now, apparently, advanced cyberweapons such as the Stuxnet virus. The U.S. defense industry's ability to produce such military capabilities on a sustained basis increases the uncertainty under which rivals must plan, compelling them to divert resources to cover a range of possible new U.S. capabilities.

After years of delay, moreover, the Defense Department is developing a family of long-range strike systems, including a new bomber. These systems will not be cheap, but it still makes sense to invest in them, because the costs to prospective enemies of countering them will be even higher and because the systems will sustain the U.S. military's ability to penetrate any enemy's defenses and threaten key targets at will. Enemies will thus have to either leave critical assets undefended or develop and field sophisticated (and expensive) defenses along their entire borders.

One key resource currently being squandered by the Pentagon is time. The quicker the United States can develop and field military equipment, the smaller the size of the standing forces it will need to maintain and the greater the uncertainty that its foes will face. Once a leader in its ability to field new systems rapidly, the Defense Department now typically requires a decade or more to do so; reducing that time frame significantly should be a major priority.

For much of the last 20 years, a relatively stable international order and generous budgets have enabled the United States to avoid making difficult choices about defense and strategy. Decisions were often dominated by the domestic politics of defense policy, parochial bureaucratic interests, and sheer inertia rather than rigorous planning. When conflict came, too often strategy ended up meaning throwing ever-greater resources at a problem and hoping that the sheer weight of the effort would enable the United States to prevail. This approach did not succeed in Afghanistan or Iraq, and it is even less attractive now that the challenges to U.S. security are growing while the Pentagon's budgets are diminishing.

Critical choices need to be made regarding the size and structure of the U.S. armed forces, their doctrine and equipment, and what are the most promising areas of future investment. The time is long past for these choices to be made consciously and intelligently, informed by a strategy based on rigorous thinking about the nature of the challenges at hand and the alternatives for addressing them that will best preserve national security.