Even complete collapse.

How would the system respond? Could the New Peace survive without its policeman? Good counterfactual analysis minimizes the number of both assumptions and alterations of reality. It is also obviously wise to choose relatively simple cases, ones that do not involve many potentially confounding variables. 127 The ramifications of an actual supervolcanic blast would not be contained in the United States; the massive amount of material ejected into the atmosphere would blot out the sun and cause global temperatures to drop for years. To keep this thought experiment manageable, let us imagine a natural disaster that only affects the United States, one resulting in the effective disappearance of U.S. military and political engagement with the rest of the world. The effect of an aloof United States on some regions need not be imagined because it already exists. In South America, the U.S. Southern Command has a minuscule operating budget and no troops to speak of, despite its theoretical “responsibility” for the entire continent. The United States maintains no significant physical presence in Africa or large swaths of Asia. A Yellowstone supereruption would presumably not change security calculations in these areas much at all. Europe would be similarly unaffected, sat least in the short term. The United States currently maintains 95,000 troops from all services in its European Command, none of whom are tasked with maintaining the internal stability of its allies. During the Cold War, U.S. troops did not involve themselves in the domestic conflicts of their host states, unlike their Soviet counterparts. Their job was always to protect Europe from without, not within. The continent is the world’s most stable, its countries the most cooperative, and its people the least martial. It would probably take more than the removal of U.S. troops for ash-cleaning duties to bring back security dilemmas, arms races, and conflict. Borders have hardened, as have norms of conflict resolution. No one can know for sure, of course, but Europe does not seem to be a good candidate for chaos in the absence of the United States. Without the presence of U.S. forces, much of the Middle East would be unstable and chaotic. With the presence of U.S. forces, much of the Middle East is unstable and chaotic. A supervolcano erupting in Wyoming would not have much impact on the security of the world’s most dangerous region. Israel would be just as safe as it was before, since its marked military superiority over all potential rivals is the ultimate guarantor of its security, not U.S. troops or ships. Without the prospect of help from Uncle Sam, the failing governments of Iraq and Libya, as well as the rebels in Syria and our allies in Saudi Arabia, Yemen, Jordan, and elsewhere, would learn to become more self-sufficient. Perhaps they would even make long-term deals with their rivals. It might be good to throw them out of the U.S. nest and encourage them to fly on their own or crash. Fears of a resurgent Iran would be articulated by the usual suspects, no doubt, but both history and the realities of power suggest Tehran would find it hard to dominate its neighbors, even if it had the will to do so. The regions that would be of most concern in such a scenario would be the peripheries of those once and potentially future great powers, Russia and China. To believers in the “deterrence model,” first described by Robert Jervis four decades ago, weakness is provocative, and the post-U.S. world would seem everywhere weak. 128 Moscow and Beijing would attempt to expand their influence, and ultimately perhaps their borders, once they were assured that they would face no pushback from Washington. Perhaps gradual interference in their near-abroads, such as we have already seen in eastern Ukraine, northern Georgia, and the South China Sea, would occur with increasing frequency in the vacuum left by a U.S. withdrawal. While such expansion cannot be ruled out, especially in the long run, large border adjustments would probably not occur in the absence of U.S. power, for least two reasons. First, the removal of American troops would not alter the calculations regarding the costs and benefits of conquest in the twenty-first century. Although absorbing neighbors sometimes paid substantial dividends in the pre–information age, today territory is unrelated to wealth. 129 The people of larger states are not automatically better off than those of small ones. India is not richer than Singapore; Russia would not benefit from invading Ukraine; China would hardly be materially better off if it ruled Taiwan. The other members of the international system might not be able to stop such adventurism militarily, but they can certainly punish it economically. The costs related to invasion and the inevitable problems that arise during occupation would outweigh any possible benefits that may accrue. Conquest in a trading system is profoundly irrational, and the incentives for peace are strong. Rational calculations are not the only motivations for cross-border violence. As Norman Angell argued a century ago, people have to believe that war is not worth the cost before they will forswear it. 130 The quest for glory and prestige has sent many an army into motion over the centuries; Alfred Thayer Mahan responded to Angell’s rationalism a century ago by pointing out that “nations are under no illusion as to the unprofitableness of war itself” but honor often compels them to fight anyway. 131 By 2017, however, those calculations have changed. It is not at all clear that glory still automatically accompanies conquest. The second reason to believe that Russia and China might not dominate their near-abroads in an essentially U.S.-free world is that the behavioral norms of the New Peace discourage aggression. Imperialism invites opprobrium, not admiration. This does not mean that such assaults could not happen—Genghis Khan was unconcerned about opprobrium, for instance, and Vladimir Putin might be too—but surely it is significant that conquest has been all but absent since the Second World War. The unipole is not the only thing restraining potential combatants; both their material and reputational interests do so as well. If and when a catastrophic supervolcanic eruption weakens the United States, other countries would still have substantial interest in maintaining the overlapping network of international economic and political institutions that serve the interests of all members. All would want to see free trade and investment continue unmolested, whether or not the global policeman could punish violators. Most would continue to place some value on international law, human rights, and the UN system. Why any state would want to move backward to a mercantilist time of pure self-help and violence would be difficult to imagine. It is 2017, not 1717. Volcanologists assure us that someday Yellowstone will awaken with terrifying fury. The human and material cost will be immense, but the ramifications for international security may not be as dramatic. While it might take that kind of event to settle the questions concerning hegemonic-stability theory once and for all, we can still use our imaginations to anticipate the kind of reaction that the system would have if the global 911 is taken off the hook. Even more decisively than a Trump superpresidency, a supervolcano eruption would test the New Peace and settle forever debates over the importance of unipolarity. Until then, one can only imagine what the system would be like without the United States. And the smart money would be with those who say that it would probably look pretty much the same, with very small amounts of conflict and warfare, even if few people seem to notice. In the end, what can be definitely said about the relationship between U.S. power and international stability? Probably not much that will satisfy partisans. The pacifying virtue of U.S. hegemony will remain largely an article of faith in some circles in the policy world. Like most beliefs, it will resist alteration by logic and evidence. Beliefs rarely change, so debates rarely end. For those not yet fully converted, however, perhaps it will be significant that corroborating evidence for the relationship is extremely hard to identify. If indeed hegemonic stability exists, it does so without leaving much of a trace. Neither Washington’s spending, nor its interventions, nor its overall grand strategy seem to matter much to the levels of armed conflict around the world (apart from those wars that Uncle Sam starts). The empirical record does not contain much support for the notion that unipolarity and the New Peace are related. At the same time, three common psychological phenomena suggest that hegemonic stability is particularly susceptible to misperception. U.S. leaders probably exaggerate the degree to which their power matters. Researchers will need to look elsewhere to explain why the world has entered the most peaceful period in its history.