THE HALLMARK of the Donald Trump administration’s National Security Strategy (NSS), which was released in December, is the idea of “principled realism.” This marks a decided shift from the policies of Trump’s two immediate predecessors. President Bush, with his expansive freedom agenda, set a high priority on the use of U.S. power to promote liberty and democracy. President Obama, less committed to the idea of American exceptionalism and more doubtful of the value of the American role in the world, sought to disengage and “lead from behind.” The Trump NSS, by contrast, represents a return to realism.

In his valuable book Realpolitik: A History, the British historian John Bew explains that realpolitik, or realism, was not, as often characterized today, an amoral approach to foreign policy. Instead, it arose in the nineteenth century in Germany, where figures such as Ludwig von Rochau, who believed in classical liberalism and who participated in the 1848 revolution, struggled to define a foreign policy that was true to their ideals but also cognizant of the limits of their power vis-à-vis states and empires ruled by authoritarians. As Bew puts it, realists focused on the question of “how to achieve liberal enlightened goals—which included balance and equilibrium—in a world that did not follow liberal enlightened rules.”

How has the Trump administration sought to resolve this dilemma? First, it has avoided the trap of equating strategy with a wish list of goals, disconnected from the realities and limits of power.

In the past, the NSS document has often consisted of bottom-up aggregations of policies, which were essentially just stapled together and labeled a strategy, but did not comprise actual conceptual frameworks or orderings of priorities. The administration deserves credit for undertaking this strategy process early on.

In fact, the new NSS is the first serious top-down, government-wide strategy produced since the well-known Defense Policy Guidance (DPG) produced by the Pentagon in 1992, after the end of the Cold War. While the DPG—which I played a key role in formulating—was not embraced as a national-security strategy by the George H. W. Bush administration, many of its concepts shaped subsequent strategy documents and policies.

Second, the Trump administration took a clear-eyed look at world politics and at how we are currently positioned. In 1992, we had arrived at a unipolar moment at the end of the Cold War. The United States was preeminent, and the issue was how to consolidate that preeminence, preclude the rise of a global rival and prevent the domination of critical regions by hostile powers. That brief unipolar era is over.

Today, in critical regions, the mission of interstate competition has returned. China, Russia and Iran now present competitive challenges to the United States and its allies and partners. Beijing, by virtue of its dynamic economy, military buildup and assertive maritime claims, is the primary strategic competitor. Russia seeks to dominate its periphery. North Korea not only seeks coercive leverage over South Korea and Japan, but also is on the verge of being able to threaten American population centers with nuclear weapons. Also, regional and global trends have made many regions unstable, producing streams of refugees that in turn have had major political impacts. Threats from nonstate actors such as international terrorists, at times sponsored by hostile states, have grown.

Third, implicitly and sometimes explicitly, the new NSS identifies strategic errors by which the United States has contributed to today’s challenges. At times the United States has recklessly disengaged, while at other times it has rushed forward with inadequately thought-through plans, or failed to take advantage of the golden hour presented by its actions. In still other instances, the United States has failed to preclude emerging threats in a timely manner. Often, it naively believed that the political systems of adversaries would converge with American values, or that generous arms-control or trade deals would induce other countries to follow the U.S.-led international order. These mistakes crossed party lines and involved both the executive and legislative branches. As a result, the balance of power in key regions shifted toward adversarial powers.

Fourth, the Trump administration has struck a new, cogent position in the perpetual debate about whether the United States’ foreign policy should be guided by the national interest or the country’s values and ideals. The Trump doctrine, reflective of a nation that is questioning the wisdom of some recent international ventures, suggests that the balance has shifted in favor of interests.

As a member of the board of the National Endowment for Democracy, I was forcibly struck by the document’s demotion of democracy promotion. Unsurprisingly, given the president’s views of the wars in Iraq and Libya, regime change is out for now. Challenging authoritarian regimes about their treatment of their own people also seems to command less importance, though the president has spoken forcefully about the situations in Venezuela and Iran.

However, the document does tout the need to provide the world with an inspirational example and collaborate with like-minded aspiring states. Trump’s approach is to enhance America’s success—and hence its example—while respecting other states’ sovereignty to chart their own political destinies. He will speak to the aspirations of others to achieve freedom—thereby standing for American principles—but acknowledge that the United States cannot right all the world’s wrongs. This is the heart of “principled realism.” It means a policy of advancing values, but with greater prudence.

THE UNDERLYING logic of the Trump administration’s diagnosis of the competitive nature of world politics and recent negative shifts in regional balances of power leads to a set of appropriately ambitious lines of action.

The first focuses on the centrality of improving the underpinnings of U.S. power, especially the country’s economic performance. This is key, as the United States cannot sustain its relative power position in the world if the foundations of its power atrophy.

On the economic front, the United States faces the fiscal challenge of supporting entitlements, social programs, and defense and international-affairs spending, at a time when the national debt and annual deficits are already at historically high levels. We will not be able to manage this challenge without higher rates of economic growth. Whether it will be possible to reach and sustain 3 or 4 percent growth—the president’s goal—remains to be seen. However, if we stay at 2 percent or less, the United States will likely be forced to choose between domestic social programs and the United States’ world role, which is not a desirable choice to face. The focus of the NSS on the U.S. economy is sound.

In terms of defense, the document rightly calls for restoring U.S. military overmatch against potential adversaries, to achieve “peace through strength.” National Security Adviser H. R. McMaster is often quoted as saying that, since the end of the Cold War, any time the United States entered a military conflict it was not a fair fight, and that preserving this overmatch must be a key security goal. This preeminence is eroding and needs to be restored. To do so, we will have to spend more, and use those resources wisely.

The vital importance of military balances is something Americans like to wish away. The 1992 DPG called for the United States to maintain a military capability that would discourage the rise of a peer competitor. Unfortunately, post–Cold War administrations cut U.S. capabilities by too much, opening the door for rising powers to pursue favorable balances of power at the regional level. At the same time, the diffusion of military technologies, particularly those of the precision-guidance revolution, means that America’s rivals are leveling the playing field. The U.S. response has been to whistle past the graveyard, defining down defense requirements and then capping this shortsightedness with the Obama-era sequestration that has badly undermined readiness and modernization.

The new National Defense Strategy, which appropriately derives from the NSS, provides a bracing description of the need to reset and bolster U.S. forces for a complex environment that involves the return of great-power competition, the need to deter and counter rogue regimes, and the imperative of continuing the fight against transnational terrorists. Every domain is now contested, and the homeland is not a sanctuary.

The unclassified version of the strategy does not describe the administration’s force-sizing criteria in detail. However, it states that in peacetime U.S. forces must be able to deter aggression in three key regions—Europe, the Indo-Pacific and the Middle East—as well as degrade terrorist and WMD threats and compete below the threshold of war. In wartime, the strategy states, the fully mobilized force must be capable of “defeating aggression by a major power; deterring opportunistic aggression elsewhere; and disrupting imminent terrorist and WMD threats.” In peace and war, U.S. forces must maintain strategic deterrence and homeland defense.

The unclassified version also fails to provide detailed descriptions of the new operational concepts or technologies that will be needed, but it makes clear that Defense Secretary James Mattis intends to shape up the Joint Force for a new era. The emphasis is on increasing the lethality of the force, though the United States will also have to come to terms with shortfalls in capacity and numbers. Mattis has highlighted the need to reform the Defense Department and specified what he expects from reform.