Over the past 18 months, the Pentagon has been pursuing a radical change in U.S. defense strategy. The Department of Defense has been working to overhaul the “two-war” defense strategy of the past quarter-century, in favor of one that focuses on winning a single high-stakes fight against China or Russia. This one-war strategy is rooted in an entirely correct judgment that defeating a great-power adversary would be far more difficult than anything the U.S. military has done in decades. Yet it also runs the risk that America won’t have enough military power to deal with a world in which it could face two or more major threats at the same time.

During the post-Cold War era, the U.S. military had a force-planning construct (a scheme that matches the size and capabilities of the force to the key scenarios it is likely to face) focused on fighting two major regional contingencies more or less simultaneously. The idea was that the United States should be able to decisively defeat an adversary in the Mideast — Iraq or Iran — without fatally compromising its ability to take on North Korea. This two-war capability was deemed critical to preventing opportunistic aggression by one adversary while the U.S. was engaged with another, and thereby upholding a grand strategy premised on deterring war in multiple regions at once. The two-war strategy, Pentagon officials wrote in 1997, “is the sine qua non of a superpower.”

After the onset of budgetary austerity in 2011, the two-war strategy gradually eroded as defense cuts made it harder to handle two regional adversaries at once. And after the Russian invasion of Ukraine in 2014, it was clear that the U.S. was facing a fundamentally different world, in which the country’s foremost adversaries were not inferior rogue states but major powers fielding formidable military capabilities. Add in that any war against Russia or China is likely to occur in their geopolitical backyards, and that both rivals have spent considerable time, money and intellectual effort seeking to neutralize America’s ability to project power, and the U.S. military would have enormous difficulty in winning even a single war against a great-power challenger.

In the 2018 National Defense Strategy and subsequent statements, the Pentagon thus outlined a significantly different force-planning construct. It announced that the fully mobilized U.S. military would be capable of defeating aggression by a great-power adversary, while also deterring (not necessarily defeating) aggression in a second theater. In other words, the U.S. is now building a force not around the demands of two regional conflicts with rogue states, but around the requirements of winning a high-intensity conflict with a single, top-tier competitor — a war with China over Taiwan, for instance, or a clash with Russia in the Baltic region.

There is plenty of serious thinking behind this shift. The new strategy is meant to signal unambiguously — to allies, competitors and the Pentagon bureaucracy — that the U.S. is now focusing squarely on great-power competition and the immense challenges it presents for a force that has been preoccupied with counterterrorism and counterinsurgency for nearly two decades. It recognizes that America’s military advantages vis-a-vis China and Russia have eroded gravely, and that the Defense Department will need new high-tech capabilities and creative operational concepts to defeat either country should war break out.

The new strategy therefore puts a priority on getting right those key concepts and capabilities — which are mostly still nascent — over expanding the force by acquiring more aircraft carriers, Cold War-era fighter jets and other legacy capabilities that would simply get chewed up in a fight against Moscow or Beijing. And it is meant to wrench the Pentagon away from America’s old way of war — one that relied on assembling overwhelming power in a theater and then launching the war at a time of our choosing — and toward a new way of war in which U.S. forces will have to deny adversaries the ability to seize key terrain quickly, while operating in an incredibly deadly environment.

In sum, the Pentagon’s approach is based on the idea that the U.S. has to nail down how to defeat just one great power rival before it takes on anything more ambitious than that. This is a sensible approach, but one that also entails real risks in war and peace alike. The major risk in wartime is that the world may throw more at the U.S. than the Pentagon can handle. America faces not one but two great-power rivals, not to mention the lesser threats posed by North Korea, Iran and various terrorist groups. There is a non-trivial chance that the U.S. could find itself dealing with serious military challenges in two or more theaters at the same time.

Indeed, if the U.S. has to throw most of its military at defeating a Chinese bid for dominance in the Western Pacific, another hostile power — perhaps Russia — could decide to roll the iron dice while America is committed elsewhere. And even if that second country doesn’t launch a major war against the U.S. or its allies, it could seek to gain diplomatic concessions by threatening — explicitly or implicitly — aggression at a time when Washington is ill-equipped to deal with it.

To be fair, the Pentagon and key former officials have argued that under a one-war doctrine the U.S. can still “deter” in a second theater, and that the success the military has in handling the first challenger will be critical in determining whether it faces others. Yet as the congressionally mandated National Defense Strategy Commission has noted, it is not clear how, exactly, the Pentagon plans to deter a second aggressor if it lacks the ability to prevent that aggressor from seizing key territory in the first place.

If U.S. allies significantly increase their own capabilities, or if Washington eventually defuses tensions with Moscow so that it can concentrate more exclusively on Beijing, then perhaps this two-war dilemma might be eased. But today, these are relatively distant and uncertain prospects.

The risks of being caught shorthanded during a war could also have significant peacetime ramifications. The military balance has a profound effect on geopolitical competition, even before the shooting starts. A country’s willingness to run risks and defend its interests forcefully depends on how well it thinks it will fare should things get ugly. If an American president knows that conflict with China or Russia would tie up nearly the entire U.S. military, he or she may be less willing to run the risk of conflict in a crisis for fear that it would leave Washington dangerously exposed. And if America’s competitors perceive this weakness, they may become more inclined to push the U.S. and its allies harder.

The architects of the Pentagon’s new strategy are undoubtedly right about one essential thing: America needs to craft the right mix of capabilities and concepts to beat either China or Russia before it significantly expands the military. But once that mix is determined, the U.S. will need to build — at significant expense — a larger force than can credibly prevent an aggressor from achieving its objectives in a second theater even as America is fighting in the first. For the foreseeable future, the U.S. is going to be engaged in intense, dangerous rivalries against China and Russia simultaneously. It cannot indefinitely risk getting caught with a one-war military in a two-war world.

Hal Brands is a Bloomberg Opinion columnist and a professor at Johns Hopkins University’s School of Advanced International Studies.