Welcome to the New Defense-Dominant World

Russia’s ill-advised invasion of Ukraine and the United States’ equally stupid war with Iran suggest that we have entered a new world of “defense dominance,” a world where larger and more powerful states find it hard to conquer territory or to get much weaker states to make big concessions. United Nations Secretary-General António Guterres apparently thinks so, telling the Financial Times, “If you look at what happened in recent times, the capacity of superpowers to impose their will or their strength or their force in several situations has been severely limited.”

The proximate causes aren’t hard to identify—the development of sophisticated drones and accurate missiles, as well as improved surveillance and targeting capabilities that have made it harder for large units to operate in the open where they can be targeted by hidden defenders. Similarly, expensive power projection capabilities (aircraft carriers, stealthy aircraft, and the like) are becoming vulnerable to defenders and are having a lot of trouble trying to eliminate cheaper and easy-to-conceal drones and ballistic missiles. In the hands of a resolute enemy, these novel capabilities have made it easier for even comparatively weak adversaries to mount a potent defense of their home territory and even impose costs at a distance, which in turn makes it harder for objectively stronger powers to compel them to surrender.

This situation could be especially challenging for the United States, which has grown accustomed to operating with near impunity against its adversaries and enjoying “command of the commons” from which it can project power against foes. True, these advantages didn’t enable the United States to defeat determined local adversaries—such as North Vietnam or the Taliban. But it did allow it to topple brittle and fragile autocrats like Saddam Hussein, Nicolás Maduro, and Muammar Gaddafi, as well as to win gross mismatches like the invasions of Grenada in 1983 and Panama in 1991.

The big question, however, is whether recent developments constitute a far-reaching shift in the efficacy of military power—the purposes for which it can be used successfully—and the broader character of international politics. As it happens, there is an extensive, if controversial, body of international relations theory that bears directly on this question, based on the simple but elusive concept of the “offense-defense balance.”

As developed by George Quester, Robert Jervis, Stephen Van Evera, Charles Glaser and Chaim Kauffmann, and others, offense-defense theory argues that the ease or difficulty of conquest could have profound effects on the security of states and how they related to each other. In brief, if conquest is easy, then all states are less secure by definition. Some will be tempted to try and expand, both because it is easier to do and because being bigger might improve their chances for long-term survival. Empires will be easy to acquire yet hard to maintain. Preemptive and preventive wars will be more common because waiting for someone else to start the war makes no sense when attacking is easier than defending. Arms races will be more intense, and levels of suspicion will be higher because states will worry that trusting others, limiting armaments, or making other concessions might leave them at a disadvantage that rivals will exploit. Moreover, if offense is easy, then most states will adopt offensive force postures and military doctrines, thereby reducing crisis stability and reinforcing mutual perceptions of malign intent. As Jervis emphasized in a landmark article, when offense is easier than defense, the security dilemma is especially intense.

If defending is easier than attacking, however, these effects are all reversed. Small states can deter or defeat even larger powers more readily, and amassing vast empires will be hard to do. Preemptive and preventive wars make less sense because the hoped-for gains will be harder to achieve. States can trust each other more easily, without worrying that an ill-advised concession or other acts of cooperation might leave them vulnerable to others. And if defense has the advantage and one can distinguish between offensive military postures and those designed solely to defend one’s own territory, then status quo states can adopt defensive military forces and doctrines, thereby enhancing their own security without threatening anyone else’s. The security dilemma will be attenuated, and international cooperation will be easier to sustain.

What determines the offense-defense balance? According to its proponents, the offense-defense balance is a function of many factors: mobility, firepower, the capacity for surprise, geography, diplomatic efficiency, and political cohesion. Geographic barriers like mountains, rivers, and oceans make conquest more difficult, for example, while technologies that enable rapid movement make it easier. It is harder to conquer states whose populations are unified and imbued with a powerful sense of common identity, but it is easier to take over a country where the population is divided and invaders can play “divide-and-rule.” The development and spread of modern nationalism made creating or holding distant empires more costly and helped bring the colonial era to an end. Conquest is also harder when states can easily detect preparations for an attack and when rapid means of communication make it easier to form a balancing coalition against an aggressor. And if defense is easy and most leaders understand this, then you have the ingredients for a more tranquil world, albeit not one of perpetual peace.

The bottom line is that offense-defense theory uses a single concept—whether it’s easier to conquer new territory or defend one’s own—as a master variable that shapes many features of world politics. It is hardly surprising, therefore, that it attracted a lot of attention when it was first articulated, and it shaped a lot of earlier work in the field, including some of my own.

But like any ambitious idea, offense-defense theory faced several important critiques. Skeptics argued that defining or measuring the balance was far from simple, emphasizing that most weapons could be used for either offense or defense and that it was therefore hard to distinguish between forces designed for conquest and those intended solely to defend. Furthermore, it was not clear if the offense-defense balance was a systemic property (i.e., something that applied nearly everywhere) or something that could only be analyzed on a case-by-case basis. It might be hard to conquer country A, for example, but very easy to conquer country B, because their location, geography, political characteristics, and such were vastly different. If so, then some states might be very easy to defend while others are extremely vulnerable, making it harder to draw general conclusions about world politics as a whole. Lastly, the history of warfare suggests that states frequently misjudge what will happen once a war begins, suggesting that the only reliable way to determine whether conquest is easy or not is to start the war and see what happens.

Moreover, the ability to attack or defend might change rapidly as new weapons are invented and tactics are refined. After World War I, for example, military planners assumed that a future European war would be another grinding stalemate, but Germany’s development of the blitzkrieg showed that rapid conquest was possible in the right circumstances. If the offense-defense balance could change rapidly and unpredictably, then the security dilemma would never disappear completely, and states would never be able to really relax. Even the profound defensive revolution created by nuclear weapons was quickly challenged by air and missile defenses, counterforce doctrines, and other attempts to find some way to fight and win a nuclear war. It might be nearly impossible to eliminate the possibility of unacceptable damage in a nuclear exchange, but that didn’t mean the major nuclear powers would stop trying to reach this goal.

Despite these criticisms, states are currently facing a situation in which conquering (most) foreign territories or trying to bludgeon opponents into submission is increasingly difficult and hard to do at an acceptable cost. In a wiser world, this situation would lead to a scaling back of ambitious revanchism, greater reliance on diplomacy, and more serious efforts to resolve existing conflicts through compromise and mutual adjustment. Sadly, we are unlikely to see that level of wisdom for several reasons.

First, interstate competition isn’t going to end, and rivals will keep searching for magic bullets that might restore their capacity for decisive military action. This situation is as true for weaker states as it is for the great powers. Current military developments may give Taiwan potent ways to counter a possible Chinese invasion, for example, but you can bet that military planners in Beijing are working overtime to find ways to overcome the drones, surface-to-surface missiles, mines, and other tools that Taiwan is developing. Knowing that war can occur even when it would be costly and unwise will lead some states to think about getting nuclear weapons, which are still the most reliable way for states to deter an attack (assuming, of course, that they that are able to get the bomb before someone launches a preventive war to stop them).

Second, powerful states aren’t going to stop trying to bully weaker ones, even if direct military attacks aren’t cost-effective. As we’ve now seen in several contexts, the instruments of coercion will shift to other realms: weaponized interdependence, trade wars that leave everyone poorer, attempted blockades, and even trying to rename bodies of water. Just because these tactics often fail doesn’t mean that they won’t be tried.

Third, there are still some loosely governed areas where no defensive advantage exists, and the purely military obstacles might be less daunting. Greenland is the obvious example here: Although the strategic and economic case for seizing the island is paltry and the political costs to the United States would be substantial, it probably wouldn’t be as hard to pull off as the war with Iran has turned out to be. And history warns that gullible or vainglorious leaders will continue to be vulnerable to clever briefers who convince them that their opponents are fragile and that the use of force will be quick, cheap, and yield vast benefits. Alas, even a world where conquest is objectively difficult and most states can make themselves secure without threatening others too badly may not be very peaceful—if key leaders believe otherwise.