ANALYSIS/OPINION:

Professor Eliot A. Cohen, a Johns Hopkins University historian who served as an adviser in both the Defense and State Departments, chose an unambiguous title for his latest book: “The Big Stick: The Limits of Soft Power and the Necessity of Military Force.” As if added clarity were needed, the book’s jacket shows combat boots on the ground. He argues forcefully that strong American leadership is indispensable for peace and prosperity in the world, and relying on soft power alone to provide it is unrealistic.

The introduction sets the tone for the book with a passage from a 1901 speech by Vice President Theodore Roosevelt: “We may be certain of one thing: Whether we wish it or not, we cannot avoid hereafter having duties to do in the face of other nations. All that we can do is to settle whether we shall perform these duties well or ill.”

This is the one message to which the author returns again and again: Facts are stubborn, the reality of world conflicts is not pretty, and our nation’s leaders better be prepared to deal competently and unsentimentally with the tough decisions they must make. He illustrates this point in the next chapter, “Fifteen Years of War,” in which he argues that neither George W. Bush nor Barack Obama expected to be a wartime president, but both were forced by events to order America’s warriors into harm’s way.

The author’s overview of America’s adversaries starts with increasingly aggressive China, whose rapid economic and military rise he views as the most important international phenomenon of the 21st century. Still, China has many obstacles on the road to becoming a superpower and a weak strategic position because of its border disputes with every single one of its neighbors.

As for confronting al Qaeda, ISIL and other terrorist organizations, Mr. Cohen asks for clarity of purpose: We need to state plainly that their ideology is rooted in Islam and that we are engaged in a generational

war to eradicate them. But he also believes that their barbarism limits their appeal and will eventually halt their momentum.

A chapter titled “Dangerous States” lumps together adversaries Russia, Iran and North Korea, along with nominal ally Pakistan. They are all authoritarian, willing to use force, and economically fragile. And their nuclear weapons or nuclear aspirations are central to their national defense. Pakistan, “if not a failed state, then a wretchedly constructed one,” has played a double game for a long time, especially in supporting the Taliban in Afghanistan. All four nations have a “paranoid style” in politics, with their media filled with presumed plots by enemies both foreign and domestic.

The “America’s Hand” chapter reviews the resources we can call on to oppose our adversaries. America’s military spending dwarfs that of its opponents. Since it represents today just 3 percent of our GDP (compared to 8 percent in the Reagan years), America’s strategic solvency is high. Its many alliances are a critical asset that give it “an extraordinary global logistical infrastructure.” And considering its powerful economy, positive demographics and robust political system, the odds are that America will prevail: “No other country, or collection of countries, has a better hand to play in international politics.”

At its core, this is a book about difficult decisions imposed by unforgiving facts. Diplomacy has an important place in the tool kit of statecraft, even when it requires political compromises with “odious regimes.” So does soft power, which, Mr. Cohen argues, is not always gentle: Sanctions, for example, can deprive a country’s poor of food and medicine.

But when all else fails, our leaders must make politically difficult decisions involving hard power. Like increasing military spending to at least 4 percent. Or like stationing troops for many years in areas of potential conflict, which worked well in the past: Leaving American troops for decades in Germany and South Korea helped those two war-torn nations find their way to democracy and prosperity. In the interest of global stability, today’s American politicians must find the courage to station American troops in Iraq, Afghanistan, Poland and the Baltic states.

The ultimate hard power decisions, though, deal with going to war and even doing so preemptively. The most sobering passages of the book regard pre-emptive strikes, especially necessary if weapons of mass destruction fall into “utterly irresponsible hands.”

This is a lucid book about war by a man who loves peace. Mr. Cohen, a reserve Army officer, has a son who served two tours in Iraq and a daughter in the Navy (which he does not mention in the book), so he is fully aware of the incalculable human cost of war. But he also knows that appeasing evil is not an option. Tragically, the world continues to add to what Churchill called the “dark and lamentable catalog of human crime.”

“The Big Stick” is a valuable resource for those trying to keep America’s flame of liberty burning bright in this stormy world.

• Dan Negrea is a New York private equity investor.