Chuck Hagel is a principled realist in an unprincipled realist world.

The president’s nominee to become the nation’s twenty-fourth secretary of defense has been a tenacious combatant, on and off the battlefield. While enthusiasts focus on his courage and critics dwell on his words, Hagel’s success will hinge on how he responds to the challenges that await him. In dealing with different audiences, from the president, cabinet and service chiefs, to Congress, allies and adversaries, Hagel’s mission should be to preserve both U.S. interests and its preeminent armed forces.

Since our founding, the United States has never sustained a purely realistic or purely idealistic national security policy. Rather, we have always settled on a combination of the two, a hybrid foreign policy that blends the values of John Adams and Thomas Jefferson, as well as those of Henry Kissinger and Woodrow Wilson.

Senator Hagel’s record and rhetoric speak to this kind of pragmatic centrism. He believes in America’s pivotal role in the world, but he is more honest than most regarding the limits of U.S. power. Some discern in this an excessive degree of pessimism. Yet Hagel, speaking on the eve of the U.S invasion of Iraq, was right to warn about “the traps of hubris and imperial temptation.” He was right, furthermore, that America can’t remake the world in its image; but it must not shirk from trying to make the world a better place. He is not an isolationist and understands the need to hasten slowly.

Hagel has been a consistent advocate of clear political objectives prior to the use of force. One should expect nothing less from someone who has seen the face of war first hand. Military means should advance definable, achievable and vital interests. Such an elementary idea is deeply rooted in both military strategic thinking (war is the continuation of policy by other means) and Just War tradition (the benefits from the use of force should outweigh its horrendous consequences).

These well-worn principles prompt Hagel to ask awkward questions, to think holistically about U.S. policy tools, and to search for durable solutions. He has not challenged America’s use of force from a position of pacifism, but from a conviction, gleaned from the wisdom of the ages, that great powers are defined as much by their restraint as by their action. Just as silence can lend gravity to one’s words, so, too, can the judicious non-use of force underscore the potency of one’s military might.

Yet an enlightened America does not alter an unenlightened world. While there is much good in humanity, it is hard not to be impressed by the resiliency of global disorder. Globalization, economic interdependence and soft power have barely dampened the intolerance of Islamist terrorists, the duplicity of determined nuclear proliferators and the nationalist ardor of rising powers.

Terrorist campaigns meet their demise, but terrorism persists and changes. Al Qaeda’s core has been weakened, but the recent actions of Al Qaeda in the Islamic Maghreb demonstrate a dangerous metamorphosis. This is particularly so as Islamist terrorists find fertile soil not just in old ungoverned spaces (such as Afghanistan and Pakistan’s tribal areas) but also newly fragile states (especially in North Africa and the Middle East). This threat will demand that a Secretary Hagel craft a strategy that uses both military and nonmilitary means in a concerted and comprehensive plan.

Nor should the new secretary be complacent about the persistent efforts of Iran and North Korea to advance nuclear weapons and their means of delivery. The fact that nuclear deterrence held through the Cold War, while important, is far from predictive. Lethal military systems empower rogue regimes to sow mischief short of catastrophic use, and not so short of it, as in the case of Syria’s apparent resort to chemical warfare against its own people. Forward presence, stronger regional partners, and tailored deterrence and defense will be more rather than less important in the coming years.

Realism stresses questions about major-power relations. Unfortunately, the high degree of fragmentation and competition among major powers offers scant comfort for U.S. policymakers. While Western powers teeter under the weight of debt and dysfunction, rising powers are hardly reassuring. Russia’s resurgence is predicated on realpolitik, although at least Vladimir Putin tells us what he really thinks. China’s leadership, on the other hand, offers soothing words, even while it clings to one-party rule, rapidly modernizes its military forces, and expands its covert reach through the use of cyber warfare. Asian nationalism is palpable between China and Japan in the East China Sea, and China and India appear to be on a similar trajectory across the Indo-Pacific later in this century. Engagement among major powers will be difficult, but infinitely more so if America loses its economic foundation and military primacy.

In the midst of this maelstrom, Chuck Hagel will be asked to demonstrate America’s enduring role in the world, retain America’s military might and oversee the effective use of force. His record will be measured by how well he fends off the world’s darker forces, and whether in so doing he simultaneously preserves U.S. interests and influence.

When it comes to shaping the U.S. role in the world, it is important to bear in mind how others respond to a perceived vacuum of power—however much rooted in past experience. One concern is that Hagel will be prudent to the point of being risk-averse, which in turn might encourage miscalculation and misadventure in others. Will almost every use of force resemble another potential Vietnam quagmire? Will he be too worried about how others view America? Secretary of Defense Hagel will have to check some of his natural proclivities and surround himself with senior staffers who can help him achieve a new Hagelian dialectic, one that balances prudence with boldness and defies easy prediction (akin to how a Hegelian dialectic reaches a new synthesis after positing a thesis and its antithesis).

When it comes to being CEO of the Department of Defense, Secretary Hagel will be tested as much as by what he seeks to cut as by what he seeks to keep. Fiscal austerity will pose an enduring challenge to someone keen to preserve U.S. military preeminence and prepare for future Black Swans. The secretary of defense will have to develop a rapport with senior military leaders, without whom it will be impossible to make intelligent, hard tradeoffs between current operations and future capability.

When it comes to serving among the elite National Security Council of the executive branch (and no longer as just another voice of Congress), Secretary Hagel will have to know when to threaten or use violence and how to do so with greatest utility. It was one thing for Senator Hagel to caution against assertive nationalism during the tenure of George W. Bush, but it would be an altogether different matter to do so in an Obama administration more wedded to soft power and retrenchment than to baring America’s talons.

Principled realism, therefore, will be tested in the coming months and years by a tumultuous world, by real budget constraints, and by the changing character of war. Chuck Hagel can be a great secretary of defense. But he will have to check some of his own natural instincts. His approach has helped him achieve his current status. But as secretary of defense, Hagel must realize that global trends are not quiescent, the executive branch is different from Capitol Hill and academe, and his newfound colleagues have already cornered the market on non-military approaches to our complex world.

Dr. Patrick M. Cronin is Senior Advisor at the Center for a New American Security (CNAS) and the former Director of the Institute for National Strategic Studies.