No one ever accused Teddy Roosevelt of lacking robust political instincts. As vice president, addressing the crowd at the 1901 Minnesota State Fair just days before William McKinley was shot, he quoted the proverb “Speak softly and carry a big stick—you will go far” and went on to elaborate: “Let us further make it evident that we use no words which we are not prepared to back up with deeds, and that while our speech is always moderate, we are ready and willing to make it good.”

As president, Roosevelt laid the groundwork for the transformation of the U.S. army from a frontier constabulary, and he ordered his Great White Fleet on a voyage around the world to show the flag. But despite his flamboyant personality, notes Eliot Cohen, a State Department counselor under President Bush, there was nothing reckless in his behavior as a statesman: he settled some outstanding issues with the Brits, he helped the Russians and Japanese end their war, and he opened the door to China.

Half a century later, we find Winston Churchill in March 1946 on the presidential train on the way to Fulton to deliver his historic Iron Curtain Speech. During their discussions, Harry Truman had told the leader of his Majesty’s opposition that it might interest him to know that “We have just turned the eagle’s head from the talons of war to the olive branch of peace” in the redesigned presidential seal.

To which Churchill replied that it might be better to have the head mounted “on a swivel so it can turn from the talons of war to the olive branch as the occasion warrants.” In Churchill’s view, for America to adopt a narrow posture of self-defense would be a mistake: during the war, he had reminded a Harvard audience that “the price of greatness is responsibility.”

Both statesmen appear in Cohen’s timely book The Big Stick, which considers military power and engagement in the world essential elements in American foreign policy. That was also the view of many Americans for decades following the Second World War, in which America had assumed the role as defender of the free world. Granted, there were rough patches along the way, during the Vietnam war and the hapless Carter presidency, but, thanks to the Reagan military buildup, the consensus held together long enough to consign the Soviet Union to the scrapheap of history.

But over the last decade and a half of continuous war, the mood has changed. President Obama clearly found the exercise of power politics distasteful, as belonging to some kind of primitive stage that mankind had moved beyond. And during the recent election campaign, the Republican presidential candidate saw fit to question the whole post–World War II foundation of U.S. foreign policy, instead advocating an America First position. This loss of purpose has not gone unnoticed by America’s enemies.

Over the last decade and a half of continuous war, the mood has changed.

To Cohen, America still remains the guarantor of a stable world order, but not content just to mouth the old verities of the past, he carefully dissects the arguments of those in favor of scaling back America’s engagement, reviews the lessons of Bush’s and Obama’s wars, assesses the current strength of the American hand, and gives his recommendations on how to proceed from here.

With occasional cases of overlap, the Isolationists come in five basic varieties: the sunny optimists who see the world as having become a more peaceful place and therefore no longer in need of the United States acting as its guardian; the self-proclaimed realists, who trust the balance of power to prevent catastrophe; the advocates of soft power who view this as a slower, but less risky alternative to force; the out-and-out pessimists who claim that the United States invariably makes a mighty bollocks out of everything it touches; and the economic worriers, who insist we simply cannot afford such extravagance.

One by one, Cohen picks apart their arguments. For instance, the trouble with the realists’ view of states as selfish but rational entities is that, by seeing all states as essentially alike, they ignore the role of personality and of ideology in history: as Cohen notes, “Hitler was not Bismarck with bad manners.” And by considering Iran’s ayatollahs “nationalists in turbans” rather than revolutionaries, they ignore what zealots might be willing to do in the name of their religion: “Coolly detached secularists themselves, the realists find it hard to take seriously talk of caliphates or hidden imams.” Non-state actors do not seem to figure much in their thinking. So all in all, he asks, how realistic can the world view of the realists really claim to be?

Or take the soft power crowd, which emphasizes the global appeal of American culture, and favors relying exclusively on sanctions and marshalling world opinion against those who break the international rules. What is wrong with this, says Cohen, is that with its freedoms and rights, “America makes as many enemies as it attracts.” Moreover, the very shapelessness of the concept means that “it cannot be directed with precision.” And that includes sanctions, relying on which presumes that despots care about their populations, which is not the case. Sanctions also cannot be dialed up and down according to need: “Once abandoned, sanctions can be almost impossible to restore.”

Cohen considers President Obama’s argument that the United States can no longer afford to be the world’s policeman and should concentrate on “nation building at home” to be the weakest of the lot and reminds us that the United States currently spends only just above 3 percent of its gdp on defense. During the Cold War, the figure ranged from 6 to 10 percent.

World War II produced the G.I. bill, and, during the Vietnam period, Johnson launched the Great Society, proving for better and worse that defense spending and the use of military power abroad does not rule out development at home. The great impediment to development at home, says Cohen, is not the military, but entitlements.

The case for global engagement must begin with a realization of how unrepresentative our times have been,” Cohen writes. “To assume that nuclear weapons or some overwhelming logic of international politics make statesmen cautious is to assume human beings are not capable of tremendous error.”

Left to its own devices, the world would indeed become a nasty place, and the Obama years gave a glimpse of it. As Cohen notes, it is not in the U.S. interest to see nationalism spread in a destabilized Europe as a result of the refugee flows from the Middle East. Or face the likelihood of Asian nations developing nuclear weapons in response to unchecked Chinese sovereignty claims over the South China Sea.

Left to its own devices, the world would become a nasty place.

No matter how much it may want to, America cannot seclude itself from world disorder for the simple reason that the world will not let it: by its mere existence, its values, and its prosperity, it represents an existential challenge to all manner of unpleasant regimes and movements around the globe. And while the United States cannot impose its ideas by force on the world, he writes, “at the same time, however, it cannot hope to flourish in a world increasingly hostile to those values.”

Cohen compares the mindset of the isolationists to that of Sherman McCoy, the protagonist in Tom Wolfe’s Bonfire of the Vanities who believed he could insulate himself from New York’s crime through his success and cocooned lifestyle only to find out that this was an illusion, when he and his mistress take a wrong turn back from the airport.

This does not mean that America shouldn’t learn from its recent mistakes, says Cohen. Thus the Afghanistan and Iraq missions have many features in common: both enjoyed spectacular success in their initial phases. In Afghanistan, 600 cia operatives directed local militias to put al Qaeda on the run, and, in Iraq, Tommy Franks’s forces sliced though Saddam’s army with impressive speed.

But in Afghanistan, there were no light infantry troops available to capture the fleeing al Qaeda forces, and, in Iraq, the U.S. high command had not given thought to what should happen next. As Franks stated, that was not his remit.

Thus, Cohen writes, both wars quickly degenerated into wars without discernible fronts: in Afghanistan, the loyalties of the various tribal factions remained a mystery and in Iraq, administration analysts misread the attacks on U.S. forces as performed by “bitter enders” from the former regime rather than the result of an insurgency and an incipient sectarian civil war.

To complete the picture, in both places, America allowed outside nations to interfere without having to pay a price, the Pakistanis in Afghanistan and the Iranians in Iraq, which Cohen terms “a stunning strategic concession by the United States.”

“In the wake of Iraq,” writes Cohen, “it is tempting to say that America will never engage in such efforts again. The fact is that it probably will. Politically our leaders must make it clear that stabilization remains a potential mission of the U.S. military.”

If additional proof of the importance of stabilization efforts was needed, he notes, the allied mission to topple Libya’s Colonel Gaddafi provided it. Desiring a “light footprint,” nato leaders opted for a combination of special operators and air power, but, with no ground force in place to fill the vacuum, chaos ensued. “Intervention without follow-up of some kind, as in Libya, is as likely to produce calamity as not intervening at all,” notes Cohen.

America cannot seclude itself from world disorder.

In future efforts, Cohen recommends not “full-bore nation-building,” which in Afghanistan and Iraq came close to depriving the locals of responsibility; instead, America should concentrate on developing security institutions and the basic rule of law, while leaving it to the locals to build schools and power plants. Such stabilization, needless to say, takes time: “To commit only to fighting wars lasting a few weeks or months is to fatally cripple one’s ability to use force.”

What, then, is the current state of U.S. militarily? On the plus side, says Cohen, its reach is global, having at its disposal some 600 bases of varying sizes scattered throughout

the hemispheres. It is ahead in some key high-tech areas, and it has the ability to integrate the various elements of military power on a large scale: “Its combination of quality and quantity remains unique.”

But its relative edge has been shrinking. While the wars in Afghanistan and Iraq have given American troops plenty of experience in low-intensity warfare, notes Cohen, they have also caused America to neglect a thoroughgoing modernization of its conventional forces, particularly when it comes to those needed against China and Russia. It has not faced a military peer in conventional battle in generations, he says, and here China and Russia enjoy a logistical advantage: where they find themselves close to the likely battlefields, the United States first has to get to the party.

Of these challengers, China represents the most serious long-term threat. Though currently still militarily inferior, Chinese leaders believe the United States is a power in decline and see themselves as the dominant power of the future. Accordingly, they have been aggressively building bases on atolls in the South China Sea, claiming it as its territorial waters, claims that have been rejected by the Permanent Court of Arbitration in the Hague: annually, an estimated 5 trillion dollars worth of trade sails through here.

Alarmingly, notes Cohen, whereas the U.S. military thinkers base themselves on Clausewitz, who emphasized the uncertainty of war, Chinese military writing is suffused with the precepts of Sun Tzu, who favored winning on the cheap through surprise and deception. The danger is that they someday may be tempted to deliver a surprise blow like the Japanese did at Pearl Harbor.

To counter this temptation and to shore up the confidence of its Asian allies, Cohen recommends a U.S. naval buildup and a buildup of long-range bombers. Currently, the plane on U.S. carriers is the F-35, an excellent all-round fighter-bomber, but not what is needed for the vast distances of the Pacific. This, of course, requires a bigger defense budget. While it is currently at just over 3 percent of the gdp, Cohen recommends raising it to 4 percent.

While China is the long-term threat, Russia, though set for economic decline, represents the most immediate danger. As the former commander of nato’s allied land command General Ben Hodges recently remarked, because it was busy elsewhere, America “took its eyes off the ball” in Europe, prematurely pulling back men and materiel from the European theater.

Bitter about the loss of empire, Putin’s aim is to ruin nato. Having annexed the Crimean peninsula, his actions include placing nuclear-capable Iskander missiles in Kaliningrad, the Russian enclave that borders Poland and Lithuania, and conducting constant snap exercises next to the Baltic States, making it hard to distinguish the peacetime picture from the wartime picture.

According to Cohen, the obvious answer to Putin’s sabre-rattling is to station permanent and substantial U.S. forces in Poland and the Baltics, rather than the rotating presence that was the model chosen after 1990 for fear of humiliating the Russians.

As for Iran and North Korea, for whom hatred of America is central to their raison d’être, the United States must be prepared to respond preemptively to the nuclear threat.

Completing the sobering picture is the continuous challenge posed by Islamist movements. Here, says Cohen, America must continue to take out their leaders in targeted killings, and combine this with a strategic propaganda effort along the lines of that used against the Soviet Union. But anybody who claims there are quick fixes is deluding himself: this may go for generations, something that has to be managed rather than solved.

In the final chapter, Cohen asks under which circumstances America should resort to military force. Determined to avoid repeating the mistakes of the Vietnam War, Secretary of Defense Caspar Weinberger in November 1984 set out six rules for its use: that armed forces should only be committed if the effort was considered vital to the American interest; that the commitment should be wholehearted and determined to win; and that the objectives should be clearly defined and use the force necessary to accomplish them. Furthermore, that troop levels should be continuously assessed, that the effort should have the support of the populace and of Congress, and that it should be only as a last resort.

The urgent task facing the new American administration is to restore U.S. credibility.

Though popular with the military, the problem with this, says Cohen, is that the preconditions were so stringent as to make the use of force virtually impossible, which Secretary of State George Schultz pointed out during his frequent clashes with Weinberger: “The lesson is that power and diplomacy are not alternatives,” Shultz stated. “They must go together, or we will accomplish very little in the world.”

Thus Cohen warns against any attempts to revive the Weinberger checklist in the aftermath of Iraq, but he is not adverse to providing some general, less prescriptive, guidelines. Of these, the most important is “Understand your war for what it is, not what you want it to be.” Grasping the uniqueness of one’s war, he says, means that one needs to stay clear of lazy analogies, parallels, and metaphors: “If there is one lesson to be learned from America’s wars from Korea though Iraq, it is that attempting to fit them into a template (particularly a template that excluded protracted irregular conflict) was a mistake.”

The immediate urgent task facing the new American administration is to restore U.S. credibility, which has been badly undermined in the Obama years, particularly during the Syrian conflict, where Obama spoke of red lines which he then failed to enforce. Doubts about a president’s seriousness in one area, notes Cohen, will raise doubts about his seriousness in other areas.

Restoring credibility is not something one can achieve overnight. Thus, after the hapless Carter years—one recalls Carter feigning consternation that Brezhnev had lied to him—Ronald Reagan set about restoring America’s military muscle with his massive arms program which he combined with some carefully calculated small steps, such as the downing of a couple of Syrian fighter jets over the Gulf of Sirte in 1981 and the invasion of Granada in 1983. But, notes Cohen, it was only with the 1991 First Gulf War under George H. W. Bush that America proved decisively that it was ready to use its new hardware.

Predictability and steadiness are essential in foreign policy, notes Cohen: in its shifts between weakness and strength, America displays a lack of predictability that invites adventurism and miscalculation from its enemies. As Teddy Roosevelt noted, the U.S. President must be willing—and believed to be willing—to back up his words with action. Now more than ever.