By Steven Pinker (for The Boston Globe)

In a bitter irony, a new edition of my 2011 book “The Better Angels of Our Nature: Why Violence Has Declined” is about to go to press. It is the translation into Ukrainian. The book’s second sentence warns that the historical declines it documents were “not guaranteed to continue,” and the Russian invasion of Ukraine in 2022 has brutally halted one of them. Historians call it “the Long Peace”: the decline of interstate war, especially wars between developed states, since the end of World War II.

By the standard definition — an armed conflict between national governments that kills at least a thousand people in a year — the invasion has been the only interstate war in Europe in more than three-quarters of a century (other than the brief Soviet incursion into Hungary in 1956), and the first outside Africa or the Middle East in more than four decades. If Russia annexes Ukraine, consistent with Vladimir Putin’s declaration that it is not a legitimate nation-state but “an inalienable part of our history, culture, and spiritual space,” it would be the only postwar instance of a recognized state being wiped off the map through conquest, and one of the few in which significant territory changed hands (Russia’s annexation of Crimea in 2014 being a predecessor).

The Long Peace was not inevitable. Like other examples of human progress, it was the dividend of particular brainchildren: ideas and institutions designed to mitigate the tragedies of the human condition.

One was democracy. Russia is an “electoral autocracy,” lacking the checks and balances that can inhibit a leader from dragging his country into stupid wars. The framers of democracy designed these checks as a safeguard against “tyrants.” Today we might diagnose them as “malignant narcissists,” with a grandiose craving for glory, a lack of empathy, and a petulant sensitivity to affronts.

The other irenic force is Enlightenment humanism: the conviction that the ultimate good is the life, liberty, and happiness of individuals, with governments instituted as social contracts to secure these rights. Putin cleaves instead to romantic nationalism, in which the ultimate good is the prestige of ethnic nations. Governments and strong leaders are their embodiments, and they struggle to stake out spheres of influence and rectify historic humiliations.

A week into this anachronistic war, obviously no one knows whether it will reverse the Long Peace and send the world back to an age of warring civilizations. Maybe — but maybe not.

The refusal of the United States and European Union to send its armies to meet Russia’s on the battlefield, very different from the responses that launched the world wars, means that the other zeroes that defined the Long Peace — no nuclear war, no great-power war, no wars between rich countries (Ukraine being the poorest in Europe) — will probably continue.

And the pacifying restraints may yet kick in. Russia is enmeshed with the global economy and will feel the pain of sanctions that were swifter and severer than predicted. It is being booted out of a wide swath of organizations it wants to belong to, from the Organisation for Economic Co-operation and Development to the Eurovision Song Contest. And Putin’s middle-fingering of the norm against wars of aggression is earning him the reputation not as a great man in history but as a deranged thug.

Can shunning by the community of nations make a difference? Though populist nationalists pretend that globalization is a passing fad, our irreversibly interconnected worldis likely to punish any country that tries to go it alone. Many of a nation’s challenges don’t respect lines on a map, and membership in an international problem-solving community will be necessary to deal with them. They include trade, technology, pandemics, terrorism, climate, piracy, cybercrime, and migrants, together with the desire of its citizens to work, study, and travel abroad and to enjoy the pleasures of world culture.

Also unlikely to go into reverse is the ongoing humanitarian revolution and its decimation of barbaric customs. Past centuries saw the abolition of human sacrifice, heretic-burning, torture-executions, chattel slavery, Jim Crow, and the legal rape of wives by their husbands. In the decade since “Better Angels” was published, the data show continuing declines of other violent practices, including child abuse, female genital mutilation, forced marriage, capital and corporal punishment, land mines, and the criminalization of homosexuality.

Yet another of these barbaric customs is war. The valuation of human well-being over norms of conformity and authority will make it harder for any leader to turn his populace into cannon fodder to indulge his dreams of historical grandeur.

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For these and other reasons, even when countries have breached the Long Peace, the invasions have seldom gone well for the invader.

History is not cyclical, but it is jerky. After the biblical Israelites abandoned human sacrifice, they kept having to take measures to prevent backsliding into the pastime. France has the dubious distinction of abolishing slavery twice, the second time after Napoleon had reintroduced it. Yet the second or third or nth time was the charm: We no longer have slave auctions. Nor do we have a need for laws against burning children as an offering to Moloch.

Could Putin’s new altar of human sacrifice also turn out to be a temporary backsliding in the obsolescence of interstate war? Let’s hope so, but it won’t happen by itself. It will require that we continue to promote the forces of enlightenment that have sent violence into decline, including the valuation of human life and the norms and institutions of global cooperation.

Steven Pinker is a professor of psychology at Harvard University and the author, most recently, of “Rationality: What It Is, Why It Seems Scarce, Why It Matters.”