First it was the Europeans who sought an escape from the tragic realities of power that had bloodied their 20th century. At the end of the Cold War, they began to disarm themselves in the hopeful belief that arms and traditional measures of power no longer mattered. A new international system of laws and institutions would replace the old system of power; the world would model itself on the European Union—and if not, the U.S. would still be there to provide security the old-fashioned way.

But now, in the wake of the wars in Iraq and Afghanistan, it is the U.S. that seems to be yearning for an escape from the burdens of power and a reprieve from the tragic realities of human existence.

First it was the Europeans who sought an escape from the tragic realities of power that had bloodied their 20th century. At the end of the Cold War, they began to disarm themselves in the hopeful belief that arms and traditional measures of power no longer mattered. A new international system of laws and institutions would replace the old system of power; the world would model itself on the European Union—and if not, the U.S. would still be there to provide security the old-fashioned way.

But now, in the wake of the wars in Iraq and Afghanistan, it is the U.S. that seems to be yearning for an escape from the burdens of power and a reprieve from the tragic realities of human existence.

Until recent events at least, a majority of Americans (and of the American political and intellectual classes) seem to have come close to concluding not only that war is horrible but also that it is ineffective in our modern, globalized world. "There is an evolving international order with new global norms making war and conquest increasingly rare," wrote Fareed Zakaria of CNN, borrowing from Steven Pinker of Harvard, practically on the eve of Russia's invasion of Ukraine and the Islamic State's march across Syria and Iraq. Best-selling histories of World War I teach that nations don't willingly go to war but only "sleepwalk" into them due to tragic miscalculations or downright silliness.

For a quarter-century, Americans have been told that at the end of history lies boredom rather than great conflict, that nations with McDonald's never fight one another, that economic interdependence and nuclear weapons make war among great powers unlikely if not impossible. Recently added to these nostrums has been the mantra of futility. "There is no military solution" is the constant refrain of Western statesmen regarding conflicts from Syria to Ukraine; indeed, military action only makes problems worse. Power itself isn't even what it used to be, argued the columnist Moisés Naím in a widely praised recent book.

History has a way of answering such claims. The desire to escape from power is certainly not new; it has been the constant aspiration of Enlightenment liberalism for more than two centuries.

The impossibility of war was conventional wisdom in the years before World War I, and it became conventional wisdom again—at least in Britain and the U.S.—practically the day after the war ended. Then as now, Americans and Britons solipsistically believed that everyone shared their disillusionment with war. They imagined that because war was horrible and irrational, as the Great War had surely demonstrated, no sane people would choose it.

What happened next, as the peaceful 1920s descended into the violent and savage 1930s, may be instructive for our own time. Back then, the desire to avoid war—combined with the surety that no nation could rationally seek it—led logically and naturally to policies of appeasement.

The countries threatening aggression, after all, had grievances, as most countries almost always do. They were "have-not" powers in a world dominated by the rich and powerful Anglo-Saxon nations, and they demanded a fairer distribution of the goods. In the case of Germany, resentment over the Versailles peace settlement smoldered because territories and populations once under Germany's control had been taken away to provide security for Germany's neighbors. In the case of Japan, the island power with the overflowing population needed control of the Asian mainland to survive and prosper in competition with the other great powers.

So the liberal powers tried to reason with them, to understand and even accept their grievances and seek to assuage them, even if this meant sacrificing others—the Chinese and the Czechs, for instance—to their rule. It seemed a reasonable price, unfortunate though it might be, to avoid another catastrophic war. This was the realism of the 1930s.

Eventually, however, the liberal powers discovered that the grievances of the "have-not" powers went beyond what even the most generous and conflict-averse could satisfy. The most fundamental grievance, it turned out, was that of being forced to live in a world shaped by others—to be German or Japanese in a world dominated by Anglo-Saxons.

To satisfy this grievance would require more than marginal territorial or economic adjustments or even the sacrifice of a small and weak state here or there. It would require allowing the "have-not" powers to reshape the international political and economic order to suit their needs. More than that, it would require letting those powers become strong enough to dictate the terms of international order—for how else could they emerge from their unjust oppression?

Finally, it became clear that more was going on than rational demands for justice, at least as the Enlightenment mind understood the term. It turned out that the aggressors' policies were the product not only of material grievances but of desires that transcended mere materialism and rationality.

Their leaders, and to a great extent their publics, rejected liberal notions of progress and reason. They were moved instead by romantic yearnings for past glories or past orders and rejected Enlightenment notions of modernity. Their predatory or paranoid rulers either fatalistically accepted (in the case of Japan) or positively welcomed (in the case of Germany) armed conflict as the natural state of human affairs.

By the time all this became unmistakably obvious to the liberal powers, by the time they realized that they were dealing with people who didn't think as they did, by the time they grasped that nothing short of surrender would avoid conflict and that giving the aggressors even part of what they demanded—Manchuria, Indochina, Czechoslovakia—only strengthened them without satisfying them, it was too late to avoid precisely the world war that Britain, France, the U.S. and others had desperately tried to prevent.

This searing experience—not just World War II but also the failed effort to satisfy those who couldn't be satisfied—shaped U.S. policy in the postwar era. For the generations that shared this experience, it imposed a new and different sense of realism about the nature of humankind and the international system. Hopes for a new era of peace were tempered.

American leaders and the American public generally if regretfully accepted the inescapable and tragic reality of power. They adopted the posture of armed liberalism. They built unimaginably destructive weapons by the thousands. They deployed hundreds of thousands of troops overseas, in the heart of Europe and along the rim of East Asia, to serve as forward deterrents to aggression. They fought wars in distant and largely unknown lands, sometimes foolishly and sometimes ineffectively but always with the idea—almost certainly correct—that failure to act against aggressors would only invite further aggression.

In general, except for a brief bout of fatalism under President Richard Nixonand former Secretary of State Henry Kissinger, they were disinclined to assuage or even acknowledge the grievances of those who opposed them. (President Harry Truman and Secretary of State Dean Acheson, the architects of armed liberalism, never had much interest in bargaining with the Soviets, while President Ronald Reagan was interested chiefly in bargaining over the terms of their surrender.)

Behind the actions of the U.S. architects of containment lay the belief, based on hard experience, that other peoples couldn't always be counted on to value what the liberal world valued—prosperity, human rights or even peace—and therefore the liberal world had to be constantly on its guard, well-armed and well-prepared against the next stirring of the non-liberal, atavistic urges that were a permanent feature of humankind.

How much easier it was to maintain this tragic vigilance while the illiberal, conflict-based ideology of communism reigned across more than half of the Eurasian continent—and how much harder has it been to sustain that vigilance since the fall of communism seemingly ushered in a new era of universal liberalism, and with it the prospect, finally, of a Kantian peace in a world dominated by democracy.

For a time in the 1990s, while the generations of World War II and the early Cold War survived, the old lessons still guided policy. President George H.W. Bush and his national security adviser,Brent Scowcroft, sent half a million American troops to fight thousands of miles away for no other reason than to thwart aggression and restore a desert kingdom that had been invaded by its tyrant neighbor. Kuwait enjoyed no security guarantee with the U.S.; the oil wells on its lands would have been equally available to the West if operated by Iraq; and the 30-year-old emirate ruled by the al-Sabah family had less claim to sovereign nationhood than Ukraine has today. Nevertheless, as Mr. Bush later recalled, "I wanted no appeasement."

A little more than a decade later, however, the U.S. is a changed country. Because of the experiences in Iraq and Afghanistan, to suggest sending even a few thousand troops to fight anywhere for any reason is almost unthinkable. The most hawkish members of Congress don't think it safe to argue for a ground attack on the Islamic State or for a NATO troop presence in Ukraine. There is no serious discussion of reversing the cuts in the defense budget, even though the strategic requirements of defending U.S. allies in Europe, Asia and the Middle East have rarely been more manifest while America's ability to do so has rarely been more in doubt.

But Americans, their president and their elected representatives have accepted this gap between strategy and capability with little comment—except by those who would abandon the strategy. It is as if, once again, Americans believe their disillusionment with the use of force somehow means that force is no longer a factor in international affairs.

In the 1930s, this illusion was dispelled by Germany and Japan, whose leaders and publics very much believed in the utility of military power. Today, as the U.S. seems to seek its escape from power, others are stepping forward, as if on cue, to demonstrate just how effective raw power really can be.

Once again, they are people who never accepted the liberal world's definition of progress and modernity and who don't share its hierarchy of values. They are not driven primarily by economic considerations. They have never put their faith in the power of soft power, never believed that world opinion (no matter how outraged) could prevent successful conquest by a determined military. They are undeterred by their McDonald's. They still believe in the old-fashioned verities of hard power, at home and abroad. And if they are not met by a sufficient hard-power response, they will prove that, yes, there is such a thing as a military solution.

This lesson won't be lost on others who wield increasing power in other parts of the world and who, like Vladimir Putin's autocratic Russia and Abu Bakr al-Baghdadi's fanatical Islamic State, have grievances of their own. In the 1930s, when things began to go bad, they went very bad very quickly. Japan's invasion of Manchuria in 1931 exposed the hollow shell that was the League of Nations—a lesson acted upon by Hitler and Mussolini in the four years that followed. Then Germany's military successes in Europe emboldened Japan to make its move in East Asia on the not unreasonable assumption that Britain and the U.S. would be too distracted and overstretched to respond. The successive assaults of the illiberal aggressors, and the successive failures of the liberal powers, thus led to a cascade of disasters.

The wise men and women of our own time insist that this history is irrelevant. They tell us, when they are not announcing America's irrevocable decline, that our adversaries are too weak to pose a real threat, even as they pile victory upon victory. Russia is a declining power, they argue. But then, Russia has been declining for 400 years. Can declining powers not wreak havoc? Does it help us to know that, in retrospect, Japan lacked the wealth and power to win the war it started in 1941?

Let us hope that those who urge calm are right, but it is hard to avoid the impression that we have already had our 1931. As we head deeper into our version of the 1930s, we may be quite shocked, just as our forebears were, at how quickly things fall apart.

Mr. Kagan is a senior fellow at the Brookings Institution and the author of books including "Of Paradise and Power" and, most recently, "The World America Made."