But that overlooked the ways in which the risk of interstate war was already rising before COVID-19 began to spread. Civil wars were becoming more numerous, lasting longer and attracting more outside involvement, with dangerous consequences for stability in many regions of the world. And the global dynamics most commonly cited to explain the falling incidence of interstate war—democracy, economic prosperity, international cooperation and others—were being upended. If the spread of democracy kept the peace, then its global decline is unnerving. If globalization and economic interdependence kept the peace, then a looming global depression and the rise of nationalism and protectionism are disconcerting. If regional and global institutions kept the peace, then their degradation is unsettling. If the balance of nuclear weapons kept the peace, then growing risks of proliferation are disquieting. And if America’s preeminent power kept the peace, then its relative decline is troubling. Now, the pandemic, or more specifically the world’s reaction to it, is revealing the extent to which the factors holding major wars in check are withering. The idea that war between nations is a relic of the past no longer seems so convincing. The Pessimists Strike Back More than any other individual, it was cognitive scientist Steven Pinker who popularized the idea that we are living in the most peaceful moment in human history. Starting with his 2011 bestseller, “The Better Angels of Our Nature: Why Violence Has Declined,” Pinker argued that the frequency, duration and lethality of wars between great powers have all decreased. In his 2019 book, “Enlightenment Now: The Case for Reason, Science, Humanism, and Progress,” he wrote that war “between the uniformed armies of two nation-states appears to be obsolescent. There have been no more than three in any year since 1945, none in most years since 1989, and none since the American-led invasion of Iraq in 2003.” Optimists like Pinker held that, rather than the world falling apart, as a quick glance at headline news might suggest, the opposite was true: Humanity was flourishing. More regions are characterized by peace; fewer mass killings are occurring; governance and the rule of law are improving; and people are richer, healthier, better educated and happier than ever before. In their book, “Clear and Present Safety: The World Has Never Been Better and Why That Matters to Americans,” Michael A. Cohen and Micah Zenko argued that the evidence is so overwhelming that it is difficult to argue against the idea that wars between great powers, and all other interstate wars, are becoming vanishingly rare. Even when wars do break out, they tend to be shorter and less deadly than they were in the past. John Mueller, a senior fellow at the Cato Institute, also reasoned that the idea of war, like slavery and dueling before it, was in terminal decline, while Joshua Goldstein, an international relations researcher at American University, credited the United Nations and the rise of peacekeeping operations for helping win the “war on war.” But in recent years, a range of critics have begun to poke holes in these arguments. Tanisha M. Fazal, an international relations professor at the University of Minnesota, contends that the decline in war is overstated. Major advances in medicine, speedier evacuations of wounded soldiers from the field of battle and better armor have made war less fatal—but not necessarily less frequent. Fazal and Paul Poast, who is at the University of Chicago, further assert that the notion of war between great powers as a thing of the past is based on the assumption that all such conflicts resemble World War I and II—both are historical anomalies—and overlooks the actual wars fought between great powers since 1945, from the Korean War and the Vietnam War to proxy wars from Afghanistan to Ukraine. Meanwhile, Bear F. Braumoeller, an Ohio State political science professor, analyzed the same historical data on conflicts used by Pinker, Mueller and Goldstein, and found no general downward trend in either the initiation or deadliness of warfare over the past two centuries. What’s more, Braumoeller contends that the so-called “long peace”—the 75 years that have passed without systemic war since World War II—is far from invulnerable, and that wars are just as likely to escalate now as they used to be. Just because a major interstate war hasn’t happened for a long time, doesn’t mean it never will again. In all probability, it will. And by focusing solely on interstate wars, the optimists miss half the story, at least. Wars between states have declined, but civil wars never disappeared—and these internal conflicts could easily escalate into regional or global wars. The number of conflicts in the world reached its highest point since World War II in 2016, with 53 state-based armed conflicts in 37 countries. All but two of these conflicts were considered civil wars. To make matters worse, new studies have shown that civil wars are becoming longer, deadlier and harder to conclusively end, and that these internal conflicts are not really internal. Civil wars harm the economies and stability of neighboring countries, since armed groups, refugees, illicit goods and diseases all spill over borders. Some 10 million refugees have fled to other countries since 2012. The countries that now host them are more likely to experience war, which means states with huge refugee populations like Lebanon, Jordan and Turkey face legitimate security challenges. Even after the threat of violence has diminished in refugees’ countries of origin, return migration can reignite conflicts, repeating the brutal cycle. A Yugoslav Federal Army tank. Perhaps most importantly, recent research indicates that civil wars increase the risk of interstate war, in large part because they are attracting more and more outside involvement. In a 2008 paper, researchers Kristian Skrede Gleditsch, Idean Salehyan and Kenneth Schultz explained that, in addition to the spillover effects, two other factors in civil wars increase international tensions and could possibly provoke wider interstate wars: external interventions in support of rebel groups and regime attacks on insurgents across international borders. Immediately after the Cold War, none of the ongoing civil wars around the world were internationalized. According to the Uppsala Conflict Data Program, there were 12 full-fledged civil wars in 1991—in Afghanistan, Iraq, Peru, Sri Lanka, Sudan, and elsewhere—and foreign militaries were not active on the ground in any of them. Last year, by contrast, every single full-fledged civil war involved external military participants. This is due, in part, to the huge growth in U.S. military interventions abroad into civil conflicts, but it’s not only the Americans. All of today’s major wars are in essence proxy wars, pitting external rivals against one another. Conflicts in Syria, Yemen and Libya are best understood not as civil wars, but as international warzones, attracting meddlers including the United States, Russia, Saudi Arabia, Turkey, Iran, France and many others, which often intervene not to build peace, but to resolve conflicts in a way that is favorable to their own interests. These internationalized wars are more lethal, harder to resolve and possibly more likely to recur than civil wars that remain localized. It is not that difficult to imagine how these conflicts could spark wider international conflagrations. Wars, after all, can quickly spiral out of control. As Risks Increase, Deterrents Decline To make matters worse, most of the global trends that explained why interstate war had decreased in recent decades are now reversing. The theories that democracy, prosperity, cooperation and other factors kept the peace have been much debated—but if there was any truth to them, their reversals are likely to increase the chance of war, irrespective of how long the coronavirus pandemic lasts. Democracy is often considered a prophylactic for war. Fully democratic countries are less likely to experience civil war and rarely, if ever, go to war with other democracies—though, of course, they do still go to war against non-democracies. While this would be great news if democracy and pluralism were spreading, there have now been 14 consecutive years of global democratic decline, and there have been signs of additional authoritarian power grabs in countries like Hungary and Serbia during the pandemic. If democracy backslides far enough, internal conflicts and foreign aggression will become more likely. Other theories posit that economic bonds between countries have limited wars in recent decades. Dale Copeland, a professor of international relations at the University of Virginia, has argued that countries work to preserve ties when there are high expectations for future trade, but war becomes increasingly possible when trade is predicted to fall. If globalization brought peace, the recent wave of far-right nationalism and populism around the world may increase the chances of war, as tariffs and other trade barriers go up—mostly from the United States under President Donald Trump, who has launched trade wars with allies and adversaries alike. The coronavirus pandemic immediately elicited further calls to reduce dependence on other countries, with Trump using the opportunity to pressure U.S. companies to reconfigure their supply chains away from China. For its part, China made sure that it had the homemade supplies it needed to fight the virus before exporting extras, while countries like France and Germany barred the export of face masks, even to friendly nations. And widening economic inequalities, a consequence of the pandemic, are not likely to enhance support for free trade.