Critics of America’s alliance posture—most notably, the “offshore balancing” school—might concede this, but they would argue that the United States could then intervene again in these regions to rectify the balance. The fundamental problem with this line of argument, as the Center for Strategic and Budgetary Assessment’s Evan Montgomery has shown, is that it underestimates the profound and potentially insuperable difficulties of regaining a military foothold in a region after having first withdrawn. In any circumstance, such an attempt would involve substantial collective-action problems stemming from skepticism about the reliability and sustainability of U.S. pledges—not unreasonable concerns since Washington had previously withdrawn—and due to the reality that such alliances would be far harder to reassemble than maintain.

These might all be manageable if the military challenges were not so severe. Even in situations in which the United States is the dominant military power, reentering a contested or defended region is generally far harder than maintaining one’s strong position next to capable allies within it. It was a lot harder for the United States to fight back into Fortress Europe, for instance, than it would have been to participate in a more serious defense of France in 1940, let alone 1936. This is particularly the case in today’s unfolding military-technological era, an age in which antiaccess/area denial (A2/AD) systems are giving an entrenched power new advantages against an opponent attempting to penetrate its defense umbrella.

Thus if China or Russia could establish broader military zones of influence in Asia or Europe, these spheres of dominance would be much costlier and harder to roll back than their creation would be to resist in the first place. From a strategic perspective, it makes more sense to maintain NATO and alliances with Asian maritime states, and prevent Moscow or Beijing from being able to establish dominance over their respective regions, than to withdraw and have to fight back into those theaters when these states have had the opportunity to establish fully mature no-entry zones.

The best way to achieve this deterrent effect is not only with paper alliances, but with deep, formalized and integrated allied military postures. Much of the reason for this is the important role of appropriately forward-deployed and, increasingly, forward-stationed forces for deterrence. As Jakub Grygiel and Wess Mitchell, of Johns Hopkins University and the Center for European Policy Analysis respectively, have demonstrated, forces on the vulnerable frontiers of Eurasia serve to preclude or foreclose an enterprising opponent’s opportunities for opportunistic expansion, perpetrated through fait accompli or coup de main strategies. In the absence of forward, combat-credible deterrence forces, such approaches could otherwise appear attractive if a potential aggressor reckons that he can bring a conflict to a close before his adversary—particularly the United States—can bring to bear the full weight of his military power. This is the logic, for instance, behind Russia’s so-called “escalate to deescalate” strategy, which is designed to bring a conflict to an end at a stage before the full preponderance of NATO’s expeditionary forces can be brought into the balance. Accordingly, forward basing is crucial to enabling effective and combat-credible military postures in key regions like Europe and East Asia. Without U.S. bases in Germany or Japan, for instance, U.S. military power in these two regions would be a shadow of what it is (and can, with reinforcements from the homeland, become).

Finally, some argue that American alliances are just too expensive to be borne. But expensive compared to what? As Washington Post columnist Robert Samuelson and others have argued, financial considerations should inform and shape but not override strategic calculations about U.S. alliances. For one thing, U.S. allies, while they could, certainly should and increasingly must contribute more to their defense, do defray the costs of U.S. forces through a variety of mechanisms such as basing and host-nation support. More fundamentally, the costs of alliances and the military and other requirements they generate are certainly substantial, but within America’s resource capacity to pay. Indeed, defense and related costs represent a much lower fraction of national expenditure than they have in the past. Defense spending consumed on the order of one-sixth of GDP in the early 1950s; today it is more like one twenty-fifth.

Moreover, even under the rosiest scenarios, curtailing America’s alliance military commitments would save something in the vicinity of 1 percent of GDP, yet the savings might well be offset by higher overall U.S. military spending to prepare forces to fight their way back into overseas theaters where it would continue to have vital interests. As some proponents of offshore balancing recognize, a military that can penetrate back into key regions once rivals like China or Russia have established dominance over them would be a more demanding standard for defense requirements than today’s, likely driving up costs even without a war breaking out.

The costs of such a policy failing would be far higher than any savings gained. It was certainly more expensive to build up the U.S. military of World War II than it would have been had Washington sustained a post–World War I armed force capable of deterring or at least confining the attacks of Germany and Japan. Paradoxically, shedding alliance commitments could result in the United States having to pay higher military “insurance premiums” as the price of living in what would likely be a far more unstable and unpredictable global security environment. The logic of retrenchment, as some proponents such as Hugh White readily concede, actually would likely necessitate a higher level of military spending for countries that must now go it alone, with all the costs that unilateral military postures entail.

Fundamentally, then, the financial expense of America’s strategic architecture of alliances should only be viewed against its more unilateral alternatives, including their “life cycle” costs. Thus, genuine strategic need should drive posture; considerations of economy should inform, shape and constrain it, rather than the other way around. As Adam Smith put it, defense is more important than opulence. America does not need to sacrifice its prosperity for an effective alliance posture, but Washington should not seek to wring every penny out of it either.

AMERICA HAS developed and sustained an extraordinary network of allies whose collective strength and will pose an enduring deterrent to any state or states contemplating aggression or coercion. Certainly U.S. involvement in and leadership of this set of alliances pose risks and generate costs, but so too would abandonment of them—and these latter risks are likely higher, and considerably so, than the ones assumed by sticking with the present course. And if the ultimate recourse offered by critics of U.S. alliances to the failure of their policy of retrenchment is to fight back into these regions and redevelop these relationships—in other words, to end up at square one, but bloodied and with hard-earned credibility diminished—why, then, should the United States abandon its alliances in the first place? Rather than backing out, is it not better to tend to and improve them while making them more equitable rather than go through all the transaction costs, broken trust and pain of severing or weakening them?

But the status quo is not sustainable either. Things will have to change to remain the same. Here, both Trump and Obama have touched on real problems that need to be addressed: allies’ free riding in an era of austerity, in which standards of living have been stagnant for too many Americans; security risks that exceed in some cases their costs; and allies that appear increasingly hesitant or unwilling to fight for their own sovereignty and freedom. Dumping allies wholesale is not the answer, but neither is stasis. The solution is to redefine America’s security relationships and transform its moribund protectorate relationships into true security partnerships. This demanding task, which awaits the next president, must take place in an era of renewed competition when it comes to global security cooperation, in which current and potential U.S. allies have their own options and will pursue their own interests, and countries like China and Russia are actively seeking openings to establish their own overseas security relationships.

How, then, might America extend its alliance advantage and put it on a more equitable and sustainable footing? First, Washington needs to remind its allies, and itself, that alliances are two-way commitments, which by their nature demand quid pro quos. They will only last as long as voters in democracies like the United States and in its allied countries continue to believe in their “value proposition.” Allies doing more for themselves is not only practically useful, but also serves as a signal of their own commitment to mutual security goals, which in turn increases the value Americans see in an alliance, especially in an environment in which Americans are again being asked to assume greater risks in light of a resurgent Russia and rising China. This requires allies to raise their defense spending and more equitably share in the risks and responsibilities of the pact. NATO countries as well as Asian allies must spend more, and spend it on genuine combat-credible military forces, intelligence and internal security capabilities to address unconventional, conventional, cyber and WMD threats. The days of smaller allies spending precious resources on outsized prestige capabilities like high-end fighter aircraft and frigates that do not actually contribute to the local-security bottom line, or on token “out of area” expeditionary forces in an attempt to curry favor with the United States, are long over. Allies must adopt a back-to-basics approach that places priority, first and foremost, on shoring up their internal and territorial security, including territorial waters, airspace and communications networks. At the same time, Washington must be more disciplined in pressing allies to focus their investments on these efforts rather than pressing them to buy American systems of questionable utility.