The great foreign-policy realist Walter Lippmann once suggested that a common American practice is to take on international commitments—but not provide the necessary means to defend them. For instance, as Lippmann observed, the United States acquired control over the Philippines after the Spanish-American War, but did not really have the Navy to defend those islands at the time. This periodic gap between military capabilities and international commitments is known as the "Lippmann gap." And under the Obama administration today, by any objective measure, the Lippmann gap is pretty bad.

Here are a couple of concrete examples. The administration has adopted a policy of pivoting or rebalancing toward the Asia-Pacific. At the same time, however, it has continued to cut back the number of ships in the Navy. The two opposing directions simply do not add up. If one of the purposes of the pivot is to reassure U.S. allies in Asia and remind China that the United States is there to stay, then how can we bolster that impression while at the same time cutting back on our maritime capabilities?

Another example: just a few weeks ago, the Pentagon indicated that it would not deploy the USS Harry S. Truman to the Middle East as scheduled. U.S. naval presence in the Persian Gulf region has now been reduced from two aircraft carriers to one. What possible conclusion can the Iranian government, and for that matter our Gulf allies, reach from this announcement, other than the United States is now weaker in the region, relative to Iran? Our allies, adversaries and competitors will not simply watch what we say; they will watch what we do. And as our ships draw down or come home, they will notice.

Any administration, including Obama's, really has only three ways to close the Lippmann gap. The first is to build up military strength, so that existing international commitments are actually defensible. This is a course scorned by foreign policy doves, left-liberals, and most academics, but it is often a perfectly reasonable choice. The second option is to scale back international commitments to match limited capabilities, which is frequently trickier than it sounds, but at least intellectually honest. And the third option is simply to deny that any serious gap exists, and to hope that nobody notices. This is what might be called strategic denial, and it is the course that Obama has chosen so far.

Obama's obvious priority for several years now has been to retrench U.S. military power incrementally in order to refocus political attention on the achievement of liberal domestic-policy legacies. This involves overarching cuts to military spending. But so far, he has not really been willing to admit what these cuts might mean overseas. That is to say, his administration has not laid out an explicit and coherent defense strategy or national-security strategy based upon the assumption of defense cuts that Obama himself favors.

One place the administration might soon choose to make it downsized defense strategy explicit is in the upcoming Quadrennial Defense Review (QDR) for 2014. The QDR was originally described by Congress as having the purpose of providing a coherent, big-picture outlook on probable international-security trends, along with their logical implications for U.S. defense strategy, military budgets and force posture. The last review, released in 2010, did not exactly accomplish this goal. In fact it was unusually vague and platitudinous, even by the standards of such government documents.

Congress has mandated that the next QDR will be assessed by an independent National Defense Panel, with key appointments made by members of Congress from both parties. House Armed Services Committee Chairman Buck McKeon (R-CA), for example, has appointed Ambassador Eric Edelman and former Missouri Senator Jim Talent to the panel. These are strong appointments, and this is the leading mechanism by which Republicans will have some input in this process. A similar independent panel three years ago did good work in pointing out some of the flawed assessments of the 2010 QDR, for example by highlighting the need for greater—not lesser—U.S. naval shipbuilding, given the administration's own Asia policies. At the very least, Republican appointees on the 2014 National Defense Panel can plant a flag, by indicating what a coherent national-defense strategy might actually look like.

A strategy begins by identifying certain vital national interests, goals or objectives. It then identifies threats to those interests, arising from particular real-world adversaries. Finally, it recommends the development and maintenance of specific policy instruments, including a variety of military capabilities, to meet those threats.

It is sometimes said that we live in an age of austerity, so inevitably budgetary constraints will drive the strategy. But resources are always limited, and strategy is always about developing a coherent approach toward specific threats under conditions of limited resources. So if we simply let declining budgets dictate how we identify threats to our national interests, we're not really engaging in strategy at all. Strategy is about prioritizing and facing tradeoffs. It's about matching up commitments and capabilities, policy objectives and policy instruments, so that the two are in some kind of reasonable balance. And this is what the Obama administration has yet to do in relation to national defense.

To give some credit to the administration, its 2012 Defense Strategic Guidance began to head in the direction of greater internal coherence, relative to the 2010 Quadrennial Defense Review. For example, the 2012 Guidance shifted away from the traditional two-war standard, by which U.S. forces are prepared to fight two major regional conflicts simultaneously. Judged purely on its own merits, this shift is actually a mistake, because it more or less invites rogue states to think they might succeed with aggressive behavior while the U.S. is preoccupied in some other part of the world. Yet given dramatic cuts in defense since 2011, there are serious doubts as to whether the U.S. can maintain the traditional two-war standard. In that narrow sense, the 2012 guidance is an improvement in terms of its internal strategic consistency: it implies less with less, rather than simply trying to be all things to all people.

But additional defense cuts of some $500 billion over the next decade are entirely possible. In that case, even the downscaled national defense strategy implied in the 2012 guidance will no longer be coherent or sustainable.

Republicans today are rightly focused on getting the federal budget deficit under control, by constraining or rolling back a variety of Obama's gargantuan domestic spending projects. At times, as with the Budget Control Act of 2011, this focus has spilled over into a willingness on the part of some to cut defense spending dramatically as well. But at the end of the day, most Republicans in Congress and in the country at large understand instinctively that national defense is among the most basic responsibilities of a limited but effective government. And of all the major political groupings in this country, public-opinion polls continue to show that conservative Republicans in particular are quite literally the last group willing to surrender America's military primacy. After all, since Obama took office, domestic spending has skyrocketed, while defense has taken more than its share of budget cuts. So it is hardly the U.S. armed forces that are the primary source of current deficits. Congressman Paul Ryan recognized this in his recent budget proposal for fiscal year 2014, in which he outlined a long-term deficit reduction plan that restores defense spending to pre-sequester levels.

If in the coming years the United States implements defense cuts anything like the ones envisioned under sequestration (on top of existing cuts from the 2011 Budget Control Act), the only way to bring shrinking military capabilities into balance with international commitments will be to cut back dramatically on those promises. The United States would then be headed toward a defense strategy resembling what political scientists call "offshore balancing." Indeed in certain ways we already seem to be headed in that direction. The relative emphasis today on long-range strike capacity, special operations, drone strikes, cyber war, area denial and light-footed approaches to international-security challenges—rather than on heavy ground forces, stability operations, counterinsurgency or major regional war contingencies—is at least a move in the direction of offshore balancing. Such a strategy has always had a certain appeal, because it appears to promise national security at minimal cost. But it carries certain risks or downsides as well.

A strategy of offshore balancing, if that is where we are headed, risks signaling to U.S. adversaries and allies alike that we are not really in the game. Naturally this will reduce America's leverage abroad, diplomatically, economically and militarily. And it will make it much harder to achieve commonly stated goals of preventing aggression, succeeding in counterterrorism operations, maintaining open sea lanes and preserving a balance of power in Europe and Asia friendly to the United States and to its democratic values.

To be sure, the general public is rather weary today of numerous U.S. strategic commitments overseas. So are a significant number of conservatives and Republicans. But we need to be clear about what abandoning these commitments would mean.