Red light reflected from the carpet illuminates Russian President Vladimir Putin as he passes U.S. President Barack Obama at a group photo for the 70th anniversary of the D-Day landings in Benouville June 6, 2014. REUTERS/Kevin Lamarque Pessimism is a useful prism through which to view the affairs of states. Their ambition to gain, retain, and project power is never sated.

Optimism, toward which Americans are generally inclined, leads to rash predictions of history’s ending in global consensus and the banishment of war. Such rosy views accompanied the end of the Cold War.

They were also much in evidence a century ago, on the eve of World War I.

Then, as now, Europe had lived through a long period of relative peace, after the end of the Napoleonic Wars. Then, too, rapid progress in science, technology, and communications had given humanity a sense of shared interests that precluded war, despite the ominous naval competition between Britain and Germany.

Then, too, wealthy individuals devoted their fortunes to conciliation and greater human understanding. Rival powers fumed over provocative annexations, like Austria-Hungary’s of Bosnia-Herzegovina in 1908, but world leaders scarcely believed a global conflagration was possible, let alone that one would begin just six years later. The very monarchs who would consign tens of millions to a murderous morass from 1914 to 1918 and bury four empires believed they were clever enough to finesse the worst.

The unimaginable can occur. That is a notion at once banal and perennially useful to recall. Indeed, it has just happened in Crimea, where a major power has forcefully changed a European border for the first time since 1945. Russia’s act of annexation and its evident designs on eastern Ukraine constitute a reminder that NATO was created to protect Europe after its pair of 20th-century self-immolations.

NATO’s core precept, as the Poles and other former vassals of the Soviet empire like to remind blithe western Europeans, is Article 5, by which the Allies agreed that “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all,” triggering a joint military response. This has proved a powerful deterrent against potential adversaries.

Vladimir Putin, the Russian president, has been most aggressive in the no-man’s-lands of Georgia and Ukraine, nations suspended between East and West, neither one a member of NATO. Had Ukraine been a member of NATO, the annexation of Crimea would have come only at the (presumably unacceptable) price of war. Article 5, until demonstrated otherwise, is an ironclad commitment.

When a 19-year-old Bosnian Serb nationalist, Gavrilo Princip, assassinated the heir to the Austro-Hungarian throne in Sarajevo, on June 28, 1914, he acted to secure Serbia’s liberty from imperial dominion. He could not have known that within weeks, Austria-Hungary would declare war on Serbia, goading Russia (humiliated in war a decade earlier by Japan) to mobilize in defense of its Slavic ally, which caused the kaiser’s ascendant Germany to launch a preemptive attack on Russia’s ally France, in turn prompting Britain to declare war on Germany.

Events cascade. It is already clear that the nationalist fervor unleashed by Putin after a quarter century of Russia’s perceived post–Cold War decline is far from exhausted. Russians are sure that the dignity of their nation has been trampled by an American and European strategic advance to their border dressed up in talk of democracy, the rule of law, and human rights. Whether this is true is irrelevant; they believe it.

National humiliation, real or not, is a tremendous catalyst for war. That was the case in Germany after the Treaty of Versailles imposed reparations and territorial concessions; so, too, in Serbia more than 70 years later, after the breakup of Yugoslavia, a country Serbia had always viewed as an extension of itself. Russia, convinced of its lost greatness, is gripped by a Weimar neurosis resembling Germany’s post–World War I longing for its past stature and power.

The Moscow-backed separatists taking over government buildings in eastern Ukraine and proclaiming an independent “Donetsk People’s Republic” demonstrate the virulence of Russian irredentism. Nobody can know where it will stop. Appetite, as the French say, grows with eating.

Let us indulge in dark imaginings, then, in the cause of prudence. Here is one possible scenario: Clashes intensify between Ukrainian government forces and paramilitary formations organized by Russian fifth columnists. The death toll rises. The ongoing NATO dispatch of troops and F‑16s to Poland and the Baltic states, designed as a deterrence, redoubles anger in Russia—“a great and humble nation besieged,” a Russian general might declare.

The American president, saying his war-weary country will not seek conflict, imposes sanctions on the entire Russian oil-and-gas sector. European states dependent on Russian energy grumble; a former German chancellor working in natural gas says his country’s interests lie with Moscow. Then, say, an independence movement of the Russian minority gains momentum in Estonia, backed with plausible deniability by Moscow’s agents, and announces support for the Donetsk People’s Republic.

A wave of cyberattacks disables Estonian government facilities, and an Estonian big shot calls the Russian leader an “imperialist troglodyte trapped in a zero-sum game.” After an assassination attempt on the Estonian foreign minister at a rally in the capital, calls grow louder for the American president to invoke Article 5. He insists that “drawing red lines in the 21st century is not a useful exercise.”

Let us further imagine that shortly after the president delivers his speech, in a mysterious coincidence, a Chinese ship runs aground on one of the uninhabited Senkaku Islands, administered by Japan, in the East China Sea. China dispatches a small force to what it calls the Diaoyu Islands “as a protective measure.”

Japan sends four destroyers to evict the Chinese and reminds the American president that he has said the islands, located near undersea oil reserves, “fall within the scope” of the U.S.-Japan Treaty of Mutual Cooperation and Security. A Republican senator, echoing the bellicose mood in Washington, declares that “Estonia is more than a couple of rocks in the East China Sea” and demands to know whether “the United States has torn up the treaty alliances in Europe and Asia that have been the foundation of global security since 1945.”

The president gives China an ultimatum to leave the Japanese islands or face a military response. He also tells Russia that another act of secessionist violence in Estonia will trigger NATO force against Russian troops massed on the Estonian border. Both warnings are ignored. Chinese and Russian leaders accuse the United States of “prolonging Cold War hostilities and alliances in pursuit of global domination.” World War III begins.

It could not happen; of course it couldn’t. Peace, if not outright pacifism, is now bred in the bones of Europeans, who contemplate war with revulsion. Europe is politically and economically integrated. America, after two wars without victory, is in a period of retrenchment that may last a generation. Wars no longer happen between big land armies; they are the stuff of pinpoint strikes by unpiloted drones against jihadist extremists. Putin’s Russia is opportunistic—it will change the balance of power in Ukraine or Georgia if it considers the price acceptable—but it is not reckless in countries under NATO protection.

China, with its watchword of “Harmony,” is focused on its own rising success and understands the usefulness of the United States as an offsetting Pacific power able to reassure anxious neighbors like Japan and Vietnam. For the time being, Beijing will not seek to impose its own version of the Monroe Doctrine. It will hold nationalism in check even as the Asian naval arms race accelerates. Unlike in 1914 or 1939, the presence of large American garrisons in Europe and Asia sustains a tenacious Pax Americana.

The United Nations, for all its cumbersome failings, serves as the guarantor of last resort against another descent into horror. The specter of nuclear holocaust is the ultimate deterrent for a hyperconnected world. Citizens everywhere now have the tools to raise a cacophony in real time against the sort of folly that, in World War I, produced the deaths of so many unidentifiable young men “known unto God,” in Kipling’s immortal phrasing.

Convincing? It would certainly be nice to believe that, as President Clinton suggested in 1997, great-power territorial politics are a thing of the past. A new era had dawned, he said, in which “enlightened self-interest, as well as shared values, will compel countries to define their greatness in more-constructive ways.” In fact, the realization that the Russian bear can bite as well as growl is timely. It is a reminder that a multipolar world in a time of transition, when popular resentments are rising over joblessness and inequality, is a dangerous place indeed.

The international system does not look particularly stable. The Cold War’s bipolar confrontation, despite its crises, was predictable. Today’s world is not. It features a United States whose power is dominant but no longer determinant; a one-party China that is a rising hegemon; an authoritarian Russia giddy on nationalism and the idea of a restored imperium; and a weak, navel-gazing, blasé Europe whose pursuit of an ever closer union is on hold and perhaps on the brink of reversal.

Pacifist tendencies in western Europe coexist with views of power held in Moscow and Beijing that Bismarck or Clausewitz would recognize instantly. After the genocides in Rwanda and Bosnia, the UN General Assembly ratified the concept that governments have a “responsibility to protect” their citizens from atrocities. But in the face of Syria’s bloody dismemberment and Ukraine’s cynical dismantlement, idealism of that kind looks fluffy or simply irrelevant.

The Baltic countries are front-line states once again. The fleeting post–Cold War dream of a zone of unity and peace stretching from Lisbon to Vladivostok has died. As John Mearsheimer observes in his seminal The Tragedy of Great Power Politics, “Unbalanced multipolar systems feature the most dangerous distribution of power, mainly because potential hegemons are likely to get into wars with all of the other great powers in the system.”

In this context, nothing is more dangerous than American weakness. It is understandable that the United States is looking inward after more than a decade of post-9/11 war. But it is also worrying, because the credibility of American power remains the anchor of global security. The nation’s mood is not merely a reflection of economic hardship or the costs of war; it is also determined by the president’s decisions and rhetoric. There was no American majority for involvement in World War I or World War II—until the president set out to forge one (helped decisively in Franklin D. Roosevelt’s case by Pearl Harbor). As Jonathan Eyal of Britain’s Royal United Services Institute says, “If a president stands up and says something, he can shift the debate.”

President Obama has made clear he does not believe in military force. His words spell that out; so does his body language. He asks, after Iraq and Afghanistan, what force accomplishes. These are fair questions; the bar must be very high for unleashing military power. But when an American president marches allies up the hill to defend his “red line”—as Obama did regarding Syria’s use of chemical weapons—and then marches them back down again, he does something damaging that the world does not forget. And when Obama, in response to a recent question about whether declaring that the United States would protect the Senkaku Islands risked drawing another “red line,” gives an evasive answer, he does something so dangerous that his words are worth repeating:

The implication of the question, I think, … is that each and every time a country violates one of these norms, the United States should go to war or stand prepared to engage militarily, and if it doesn’t, then somehow we’re not serious about these norms. Well, that’s not the case.

If these treaty obligations do not constitute a red line triggering a U.S. military response—the only way to prove the seriousness of “these norms”—all bets are off in a world already filled with uncertainties. A century ago, in the absence of clear lines or rules, it was just this kind of feel-good hope and baseless trust in the judgment of rival powers that precipitated catastrophe. But that, it may be said, was then. The world has supposedly been transformed.

But has it? Consider this article in my father’s 1938 high-school yearbook:

The machine has brought men face to face as never before in history. Paris and Berlin are closer today than neighboring villages were in the Middle Ages. In one sense distance has been annihilated. We speed on the wings of the wind and carry in our hands weapons more dreadful than the lightning … The challenge of the machine is the greatest opportunity mankind has yet enjoyed. Out of the rush and swirl of the confusions of our times may yet arise a majestic order of world peace and prosperity.

Optimism is irrepressible in the human heart—and best mistrusted. Our world of hyperconnectivity, and the strains and aspirations that accompany it, is not so novel after all. The ghosts of repetition reside alongside the prophets of progress. From the “rush and swirl” of 1938 where “distance has been annihilated” would follow in short order the slaughter of Stalingrad, the mass murder of European Jewry, the indiscriminate deaths in Hiroshima and Nagasaki, and the anguish of all humanity. We should not lightly discard a well-grounded pessimism or the treaties it has produced.