Personnel from the U.S. Army and the People's Liberation Army attend a closing ceremony of a "Disaster Management Exchange" near Nanjing, China, on November 17, 2018.Aly Song / Reuters

America’s decline relative to a rising China has sparked interest among academics about power shifts in the international order—whether they can happen peacefully and under what conditions; what precedents exist and what they tell us. Now comes an important book, Twilight of the Titans, by Joseph M. Parent and Paul K. McDonald, who use quantitative analysis of power transitions to analyze the problem. What they find provides a warning to a rising China, and a road map for a declining United States to regain its standing.

The Harvard political scientist Graham Allison called the problem “the Thucydides Trap,” in which the country in relative decline so fears the rise of a challenger that it chooses to go to war to prevent it. And while Allison’s book Destined for War has its detractors, it served the worthwhile purpose of drawing us all back to Thucydides’s History of the Peloponnesian Wars and sounding the alarm that U.S. policies designed to confront China risked accelerating American decline.

One reason Allison was roundly criticized was that in order to have a large enough number of cases to write a political-science book, he created “transitions” among great powers that didn’t withstand rigorous examination. So, for example, Britain and France ceding ground to Germany after World War II is strongly conditioned by those three countries having a common security guarantor, the United States, that was substantially stronger, and that advocated for and buffeted the change. History has really seen only one peaceful hegemonic transition: Britain to the United States in the late 19th century. It remains an open question whether nuclear weapons will stabilize hegemonic transition.

One important question Allison’s book raises is whether the strongest states in the international order behave differently from other states. A hegemon is the rule setter and enforcer in the international order. It is typically (but not necessarily) the strongest power, because states fight for the right to establish terms favorable to their interests—so Britain after the Napoleonic Wars, which “ruled the waves,” could waive the rules; and the United States in the 20th century and especially after World War II became the architect of what is called the liberal international order, or the rules-based international order.

Parent and McDonald survey power transitions since 1870 (when data on gross domestic product first started being reliably collected) to explore the behavior of both the top states in the order and the lesser but still powerful states. They examine 16 cases of relative decline, some by hegemonic powers and some by mid-level states.

What they find is that most states respond sensibly to relative decline, undertaking prompt, proportionate retrenchment, because they seek strategic solvency—they don’t want to go bankrupt (and thus lose their independence). That is, the sensible policy choices that helped make them powerful also help them cope with straitened circumstances and decide to reduce their military and avoid armed conflicts. For most states, the choice of retrenchment helps them regain stature; those that fail to retrench never do.

The authors also find that states experiencing decline are not generally seen as inviting targets for aggression by others. So rising states are not generally tempted to attack a weakening rival. Parent and McDonald’s research suggests this is because the states experiencing decline steer clear of conflicts—war being the unsentimental arbiter of state power, declining states would rather not risk demonstrating their diminution. Their research also suggests that these states tend to prevail in the conflicts they do choose to initiate. Parent and McDonald conclude, “This suggests that declining powers are flexible and formidable.”

The Chinese Communist Party leadership would have benefited from Parent and McDonald’s research, showing as it does that “premature bids for hegemony can not only encourage the formation of hostile foreign coalitions but also upset the fragile domestic foundations of long-term growth.” For all the talk of China’s leaders as brilliant strategists with a hundred-year time horizon in their planning, their choices in the past decade would seem to conform to Parent and McDonald’s description of a premature bid for hegemony.

This is all good news for the United States in a time of waning relative power in the international order. If the future conforms to the data, we can expect a United States that gets its house in order while avoiding wars, as the Chinese activate antibodies against their continued rise, and thereby allow the U.S. to regain its former standing. This is not as good news for America’s allies, who will have to bear more of the cost, risk, and responsibility of defending their security and sustain an international order beneficial to their interests.

But this is where I fear the aggregate data—prioritizing a large number of cases all treated with equal weight—may lead us astray. Because two things not apparent in the numbers may prove much more important than the findings from Parent and McDonald’s study.

First, it probably matters whether a state is declining from the top spot or a spot near the top. Losing the hegemon’s spot matters an awful lot more than losing an ordinal ranking. There are many strong and important countries in the world, but the relative ranking among them is of little importance. Does Germany care whether it is the third- or fifth-largest economy? Perhaps marginally, but not fundamentally. But countries do care whether they hold the top spot, the hegemon, because that gives them the ability to set the rules of the game. If China becomes the hegemon, it will change the rules from what they have been in the time of American hegemony: Preference will replace law, small states will be dictated terms by strong states—patterns we have already begun to see in China’s intimidation of regional neighbors and predatory trade and business practices. The United States and its liberal allies may well fight to prevent those changes.

Second, the nature of the state matters more than Parent and McDonald credit. They find no correlation between regime type (that is, the nature of the state: communist, authoritarian, or democratic) and war involving a declining power. But in their 16 cases of relative decline since 1870, the United Kingdom is the declining power six times (Parent and McDonald elegantly term it “the most experienced declining power”); Germany is the rising power eight times. It may be true, as Parent and McDonald conclude, that “aggressive responses to decline appear to be the exception, rather than the rule,” but in their data, Germany is the only state to become more aggressive and authoritarian. And for Britain to provide so many peaceful instances of relative decline may just tell us that Britain is exceptionally agile in playing a weakening hand well and peacefully. What we may be seeing in their study is less a generalizable theory of the behavior of declining powers than a demonstration of British and German strategic cultures. They may both be anomalous, which makes them poor examples on which to build a theory.

Regime type may also matter much more because it speaks to a state’s resilience. Parent and McDonald find that “recovery is less likely than not, but the only road to recovery leads through retrenchment,” and that means a government must acknowledge its circumstances and choose different policies. Authoritarian states tend to be more brittle than their democratic counterparts. Lacking free media to publicize failures and challenge polices, lacking distributed power and civil society to experiment with alternatives and check excess, and lacking elections as competitions among different possible directions for policy, authoritarian governments tend to remain committed to failing policies longer.

Twilight of the Titans is a meaningful contribution to the debate about whether the decline of a great power is to be feared as a cause of war in the international system. Parent and McDonald took a big, important question and tried to find an answer by aggregating what we know about both great powers and their mid-level counterparts. It is not simply an interesting academic question; they make a very strong case that fighting preventive wars is self-defeating for declining powers. Rather than fight to prevent a rising challenger, states losing their relative power should retrench and compromise to avoid conflict. Adopting Parent and McDonald’s policy recommendations, though, would be learning to live with “democracy with Chinese characteristics.” Which, interestingly enough, is also the policy recommendation Graham Allison makes in Destined for War.

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