IN THE first century BC, the Roman historian Sallust wrote that the republic had descended into internal strife because of the destruction of its enemy, Carthage, in the Third Punic War. Fear of the enemy, or metus hostilis , produced domestic cohesion. Without an adversary, Romans turned their knives inward: “when the minds of the people were relieved of that dread [of Carthage], wantonness and arrogance naturally arose.”
Something similar may be happening today. There are numerous explanations for the current discord in the United States, ranging from globalization to the splintering of American communities. But one big factor is being largely ignored: the lack of a foreign threat. External threats can unify diverse populations. Psychologists have shown that people quickly form “in-groups” and “out-groups” (us versus them), whether it’s two sports teams going head-to-head or two nations at war. The desire for mutual protection, and sometimes for vengeance, can reduce enmity between in-group members and create a “one-for-all” mind-set.
A threatening rival can also reinforce a sense of national identity. The Harvard political theorist Karl Deutsch described a nation as “a group of people united by a mistaken view about the past and a hatred of their neighbors.” According to the political scientist Clinton Rossiter, “There is nothing like an enemy, or simply a neighbor seen as unpleasantly different in political values and social arrangements, to speed a nation along the course of self-identification or put it back on course whenever it strays.”
The role of foreign peril in cultivating a sense of national identity may be especially important in the United States. American self-identity is not based on an ancient shared heritage, but rather on a set of political ideals: the creed of individual rights and democracy. This is a fragile basis for unity in a continent-sized country populated by huddled masses from all over the world. The existence of the other may be essential to shore up American identity and reinforce a sense of political exceptionalism.
American history is a story that weaves back and forth between eras of threat and eras of safety , based on the degree of external danger. During eras of threat, the opponent is a preoccupying thought. Foreign and domestic policies are viewed through this lens of competition. Eras of threat sharpen the boundary between the American in-group and the enemy out-group. Within the boundary, there’s a heightened sense of national identity. Americans rally around the president and put more trust in national institutions. Social cohesion increases and there may be an easing of racial, political or economic divisions among those struggling together for the cause. For minorities within the in-group boundary, there can be new opportunities for social progress. Meanwhile, wars that occur during eras of threat are the titanic crusades of American history, which usually have strong public support.
But there’s also a darker side to the eras of threat. In-group unity can become suffocating conformity. In the face of foreign menace, power may be centralized in the White House, as Americans prioritize security over liberty. And woe to anyone caught on the wrong side of the boundary. Americans will be intolerant of dissent from fellow citizens who question the threat. The rights of Americans identified with the adversary may be trammeled underfoot. And the opponent is often viewed as a single malevolent entity, with little distinction made between combatants and civilians. If war breaks out, the American crusader may fight with a terrible fury, seeing the targeting of enemy civilians as justified in the pursuit of total victory and retribution against evildoers.
By contrast, during eras of safety, the United States loses its preoccupying focus and people turn their attention to domestic affairs. The in-group versus out-group boundaries start to blur. Americans are less sure of their national identity. Social cohesion may be replaced by a mood of fractiousness. People become more distrustful of national institutions, including the presidency. During these eras, military interventions are targeted against amorphous threats or humanitarian emergencies, often involve nation-building or peacekeeping operations, and tend to be unpopular.
But eras of safety also exhibit a positive side. For one thing, Americans are relatively sheltered from harm. In addition, the blurring of the in-group/out-group boundary can produce more tolerance for radical or skeptical voices. Meanwhile, military operations are less likely to be fought as zealous crusades, and there is greater self-criticism of U.S. actions, for example, facing up to the commitment of possible war crimes.
Of course, these descriptions represent broad generalizations, and the tendencies are not absolute. In each period, plenty of people don’t follow the predicted mind-set. Nevertheless, the eras capture some distinctive features of the United States as foreign threats wax and wane. A brief look at American history shows that, although eras of threat and safety do not repeat themselves, they do rhyme—with important consequences for today’s battle between Hillary Clinton and Donald Trump.