This is the third article of our “Sacking of Rome” week: red-teaming the global order and learning from history.

“Thus, what is of supreme importance in war is to attack the enemy’s strategy…Next best is to disrupt his alliances…” Sun Tzu, tr. Samuel B. Griffith

This week, we are asked to consider what might bring down America’s global hegemony, considering for comparison the threats faced by Rome’s imperium over the course of its history.

The exact historical causes of the Roman Empire’s final fall (officially in A.D. 476, with the abdication of the last emperor, but arguably in A.D. 410, with the Visigothic sack of Rome) have been the subject of dispute since at least Edward Gibbon’s famous History was published, and will not be resolved here, but it is worth looking at some of the more plausible explanations. A rough consensus emerged in the late 20^th century that by the end the Empire was bankrupt and unable to pay for its own defense. This led the anthropologist Joseph Tainter to argue the collapse happened when Rome’s subjects had the opportunity to defect to invading barbarians: the Roman state was too expensive to maintain and could not be made affordable, and the ferocious and appalling tax burden it placed on its citizens (which in extremis caused them to sell their children as slaves to pay their bills) caused them to look for any chance to join a different system; the barbarian incursions in the end were unopposed because, relative to the oppression of the Roman state, they posed less of a material threat. In Tainter’s view, the final collapse was, for the average Roman, a step up rather than down.

Despite the undeniable evidence (though we are, admittedly, working with archaeological specimens that are literally fragments and a handful of literary sources) that Rome was experiencing financial troubles throughout the era of the Empire (which caused it to debase its coinage in an attempt to get through seignorage what it could not through taxation), there is a contrary view, articulated most recently by the Oxford historian Peter Heather. In Heather’s argument, the Empire was doing relatively well financially at the end (the really burdensome taxation may have occurred more in the constant civil war of the 3^rd century A.D. than the more peaceful 4^th century, and may have had more to do with the depredations of marauding armies than the peacetime needs of the state), and was finally experiencing some domestic tranquility and normalcy. What brought it down was not its own internal rot, but a few well-timed heavy blows: just as Rome was having to arm itself to deal with a resurgent Persia, the Huns arrived in Europe, pushed Germanic barbarian tribes southward, and the combination of this and ineffective Roman diplomacy led to barbarian armies knocking at the gates. In Heather’s formulation, the final crash was simply the result of Germanic tribes operating in larger groups with larger armies in the field than they had previously, having been pushed in that direction by the Huns. Rome could not withstand the pressure, and it was defeated on the battlefield.

One could also point to the classic argument, which began with Edward Gibbon, that Christianity had made Romans less warlike, and that this, in turn, had made the Empire easy prey. This has been questioned in more recent times, but it may have had some effect.

Whatever brought Rome down, one can come up with a reasonably good synthesis of these proposed factors as a means of understanding what challenges await the U.S. One need only suppose that these explanations were all, to some extent, correct. Put in simple terms, the recipe for the downfall of imperium involves increasing need for defense spending, structural inability to cut costs, and a generalized apathy on the part of those within the “empire’s” bounds, combined with a changing geostrategic environment and war weariness at home. A few good, sharp knocks are then quite enough to bring it down. If one were looking for trouble for the U.S. in the near future, this is what one would look for.

At present, the U.S. is having difficulties maintaining its primacy and hegemony. Its defense budget, relative to its GDP, is in decline. Somewhat like Rome, its internal governing structure makes it difficult for it to avoid waste and intelligently allocate resources: at the moment, its political system is near-paralyzed, and whomever one may blame for this state of affairs, this makes it that much more difficult for it to respond to a changing strategic environment. At the same time, as I have noted in past posts, the U.S.’ geopolitical rivals have been rearming; one need only note the ongoing political maneuvers in the East and South China Seas and Russia’s incursions into Ukraine to discover that these rivals not only possess more power, but are increasingly unafraid to use it. In the face of these developments, the U.S.’ allies have remained apathetic: all of them likewise have internal political dynamics (the tradeoff between welfare benefits for aging populations and rearmament) that make it difficult for them to decide to do more, and in the case of Ukraine, economic incentives work against their involvement. The perception of U.S. weakness and reluctance to protect allies has not helped this dynamic.

The sharp shocks might come in the form of a series of crises in which the U.S. was unable or unwilling to act as the global guarantor that it claims to be. Russia under Putin is widely believed to have designs on the Baltic states, which on the one hand are demilitarized and notoriously difficult to defend, and on the other are NATO members that the U.S. is obligated by treaty to protect (the classic “can’t/must” dilemma personified). And while China’s actions with regard to the Senkakus, the Paracels, and (for that matter) Taiwan have to date mostly involved mere posturing, it is easy to imagine a future scenario in which China’s leaders were forced onto a more hawkish and nationalist path by factional rivalry within the ruling Party or by economic stagnation resulting in the need to distract its population.

What if a series of military moves by China and Russia happened against multiple U.S. allies at the same time? The so-called “two war doctrine” is now a relic, but the U.S. military’s capabilities are formidable, and it might be able to respond to attacks on, say, Estonia on the one hand and Taiwan on the other. If caught unprepared, however, it might be forced to cede initiative at least temporarily in one or more theatres, which might be enough time for either China or Russia to turn its takeover of a U.S. ally into a fait accompli. In effect, there is no guarantee that U.S. forces would be in position to stop an aggressive move before it was made and before it initially succeeded. At that point, the U.S. would face not only the cost of mobilizing for war (particularly if the military’s existing resources were inadequate to the task of retaking the lost territory), but also the risks associated with initiating or renewing a major conflict with two nuclear-armed great powers at the same time, possibly in the absence of immediate and substantive assistance from allies. Depending on the U.S. leadership, political situation, and public mood at the time, it is easy to envision political factions uniting around a dovish policy response, possibly with negotiation or ineffective sanctions used as a face-saving measure.

The consequences of such a policy would be disastrous for the U.S.’ international political position. Although the U.S. would retain its economy and (presumably) its armed forces, its allies would quickly make their own arrangements: a great power guarantor that has been shown to be uninterested in one’s protection is at best useless and at worst an unacceptable risk, and states that had previously relied on the U.S. to protect them from Russia and China might decide that it was safer to appease Russia and China. In two major geopolitical regions, the U.S. might quickly find itself friendless and alone.

Truly destroying a great power is difficult, but knocking it off its political perch can be done given the right mix of initiative and opportunity. The American equivalent of the Fall of Rome would be a world in which Americans awoke one morning to discover that they were no more influential than anyone else, and a good bit less than some in some places. The Pax Americana, like the Pax Romana, would give way to something new.

It must be stressed that this scenario is at the moment far-fetched, and far from inevitable. Avoiding it, however, will require a renewed commitment on the part of the American public to putting up the resources necessary to fulfill the role they want their country to play. A dose of political realism and willingness to compromise for the good of the country would not come amiss, either. Or, in Lord Macaulay’s memorable words about America’s mighty world-ruling predecessor: “As we wax hot in faction, in battle we wax cold.”

Martin Skold is currently pursuing a PhD in international relations at the University of St. Andrews, with a thesis analyzing the strategy of international security competition.