On January 20, the U.S. Department of Defense released the Trump administration’s National Defense Strategy, followed nearly a month later by the president’s budget request for fiscal year 2019. Happily, the two are reasonably well aligned, an outcome that is never assured given how disconnected the processes of strategy formulation and budget building can be. Both documents clearly prioritize strategic competition with China and Russia. But good strategy involves decisions not only about what to prioritize but what not to. Unfortunately, neither document makes clear what missions the Department of Defense is going to end or deemphasize in order to shift focus to this new and very resource-intensive top priority of “expanding the competitive space” against countries investing heavily in high-end capabilities designed to limit U.S. freedom of action (known as “anti-access/area-denial” capabilities). This omission is a problem. As large as the expected increase for defense spending is in fiscal years 2018 and 2019, it is still not enough to cover everything.

In his remarks rolling out the request for the increase, David L. Norquist, the chief financial officer and controller at the Department of Defense, emphasized that this new budget was shaped by the new strategy, despite its concurrent construction. How large the defense budget needs to be depends on what the body politic wants the force to be able to do, which is the subject of the strategy. Budgets are not strategic documents, but when done correctly they are a necessary component of making a strategy real.

And this strategy wants the armed forces to do a lot. Day to day, they must “deter aggression in ... the Indo-Pacific, Europe, and the Middle East; degrade terrorists and WMD threats; and defend U.S. interests from challenges below the level of armed conflict.” In times of war, the mobilized force must “be capable of: defeating aggression by a major power; deterring an opportunistic aggression elsewhere; and disrupting imminent terrorist attacks and WMD threats.” In addition, the force must always defend the homeland and deter strategic attacks. This guidance represents an expansion of the Obama administration’s first defense strategy, the 2010 Quadrennial Defense Review, which at the time was criticized for being overly broad and underresourced.

Budgets are not strategic documents, but when done correctly they are a necessary component of making a strategy real.

Nothing in the unclassified summary of the National Defense Strategy indicates that the Pentagon will be reducing any of its current commitments—indeed, by all appearances they are increasing. In fact, when asked during his rollout of the strategy whether the administration would be decreasing its focus on counterinsurgency—a time- and manpower-intensive mission—in order to shift resources to strategic competition with China and Russia, Secretary of Defense James Mattis said no. Invoking the academic strategist Colin Gray, he noted, “the enemy will always move against your perceived weakness. We cannot … say we’re not going to do counterinsurgency, because you know what’s going to happen. And so we’re going to have to do it.”

In sum, Trump’s defense strategy dictates that the military must be able to operate continuously in three diverse geographic regions. At the same time, it needs to be investing in advanced capabilities and sustaining the readiness required to defeat a major power and combat terrorist organizations, all the while keeping the United States safe from weapons of mass destruction delivered by either state or nonstate actors. This set of missions brings with it some very large bills. To make this strategy real, the department must increase investments in developing and fielding the high-end capabilities required to retain the United States’ technological edge against China and Russia. It needs to modernize the aging nuclear triad. And it must sustain a force large and well-equipped enough to continuously operate in three diverse regions in ways that meaningfully deter would-be aggressors and combat terrorist threats.

Still, the fiscal year 2019 defense budget request provides $686 billion in total, resources that can cover an awful lot. The largest increases by percentage are in the research, development, test, and evaluation and procurement accounts—15 and 11 percent, respectively. These funds go toward capabilities such as missile defense; advanced aircraft and ships such as the F-35, P-8A, and DDG-51; and next-generation space systems. These investments make sense to support a strategy that prioritizes strategic competition with Russia and China, who are investing heavily in advanced capabilities that are designed to limit U.S. freedom of action in their regions. The budget also sustains plans to modernize the nuclear triad and makes necessary improvements to U.S. military space architecture, which are required because space is now a contested environment. Lastly, the budget repairs broken glass left by nearly a decade of budget instability and cuts made across the board, rather than guided by strategic priorities. These repairs include: increasing numbers of service members to fill out understaffed units; replenishing depleted stocks of precision-guided munitions; and sustaining military bases, airfields, and training ranges that have fallen into disrepair. But it is still not enough to cover everything the strategy wants the force to be able to do.

The midterm budget plan, known as the Future Years Defense Program, announced in this proposal is the most concerning disconnect between the expansive strategy and fiscal reality. It projects that for the years 2020 through 2023, the defense budget requests will grow only at the rate of inflation—and even then, it uses a highly optimistic assumption for inflation rates given current trends, which are increasing. With this projection, the Defense Department will start each budget year already behind the curve—personnel and maintenance costs grow faster than the rate of inflation, due to factors like the increased cost of health care economywide and increasingly complex military platforms with longer life spans. Put all these factors together, and it is likely that, after this initial increase in fiscal year 2019, the defense budget will actually shrink in real terms for the next several years. If current defense leaders are pinning their hopes on back-office reform to generate savings that can cover this gap, they are likely to be disappointed, as so many of their predecessors have been.

The bottom line is that U.S. strategic ambition now exceeds resource reality. But then again, it usually will. Leaders in the national security space manage this gap by making calculated decisions about where to accept risk. It is not enough for a strategy to announce clear priorities, as this one has done. It must also clearly tell the department what it should stop doing, or do less of. We can hope that the classified National Defense Strategy gives the Pentagon explicit guidance in this vein. If defense leaders do not make some difficult decisions, the result will be a force that is stretched too thin. Tough choices are less tough when budgets are growing, but leaders still have to make them. Otherwise we will continue to experience the readiness challenges the force is currently facing, and worse. Units risk spending so much time deployed that they do not have adequate time for maintenance and training, one of the likely factors in last year’s navy ship collisions in the Pacific. The result could be a force with little ability to surge in response to a crisis.

Meanwhile, just as the Defense Department is feeling pretty flush, especially compared to recent years, this same president’s budget request slashes funding for much of the rest of the U.S. foreign policy apparatus. A budget that increases defense spending while gutting other modalities of U.S. power is not a recipe for sustained U.S. global leadership. In 2013, when then General Mattis said, “if you don’t fund the State Department fully, then I need to buy more ammunition,” he was not describing an optimal approach to U.S. foreign policy. He was stating what used to be common knowledge—that continued U.S. preeminence requires not only a strong military but also a full diplomatic toolkit.

CORRECTION APPENDED (February 21, 2018): An earlier version of this article reported that the research, development, test, and evaluation and procurement accounts of the 2019 defense budget request had increasted by 13 percent and 10 percent, respectively. In fact, they grew by 15 percent and 11 percent, respectively. We regret the error.