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.

DURING THE late eighteenth century, the American colonies were divided by a wide range of social and economic issues, and were protective of their individual identities and sovereignty. But the British threat caused these disparate colonies to band together. As John Jay wrote in Federalist No. 2, “Well-grounded apprehensions of imminent danger induced the people of America to form the memorable Congress of 1774.” The Declaration of Independence in 1776 recounted Britain’s “repeated injuries and usurpations, all having in direct object the establishment of an absolute Tyranny.” The colonies formed an imperfect alliance, squabbling over contributions of money and men. Nevertheless, the degree of cohesion achieved was roughly proportionate to the perceived danger. “Join, or Die,” said the revolutionary-era slogan. Or, as Benjamin Franklin put it, “we must, indeed, all hang together, or most assuredly we shall all hang separately.”

After independence was achieved, foreign threats motivated the push for a strong central authority, which culminated in the creation of the U.S. Constitution in 1787. Spain closed the Mississippi to American navigation, and Britain retained armed posts along the Great Lakes in violation of the peace treaty of 1783. Pirates threatened American shipping and Native Americans attacked settlers in the Northwest. In Federalist No. 4, Jay wrote that independent colonies would be unable to create armies and fleets, whereas a “Union” could “apply the resources and power of the whole to . . . defense.” The era of threat also produced a crackdown on the civil liberties of perceived enemies at home. In 1798, fear of France led to the Alien and Sedition Acts, which allowed the deportation of noncitizens who came from hostile nations, and prohibited the publication of malicious attacks on the president or Congress.

During the early decades of the nineteenth century, the era of threat gave way to an era of safety. The United States achieved a position of strategic immunity, protected by large oceans, and strengthened by a rapidly growing population and economy. As Abraham Lincoln remarked in his 1838 Lyceum Address,

All the armies of Europe, Asia and Africa combined, with all the treasure of the earth (our own excepted) in their military chest; with a Buonaparte for a commander, could not by force, take a drink from the Ohio, or make a track on the Blue Ridge, in a trial of a thousand years.

But it was precisely this lack of an enemy that laid bare sectional division and smoothed the road to civil war. In the same speech, Lincoln predicted, “If destruction be our lot, we must ourselves be its author and finisher.” Without a foreign danger to occupy their thoughts, Americans focused on domestic fissures, especially over slavery. On the eve of the Civil War, the U.S. secretary of state even suggested declaring war on France and Spain to deliberately create a unifying enemy and ward off disunion. If the United States were attacked by a foreign power, “the hills of South Carolina would pour forth their population to the rescue of New York.”

With the assault on Fort Sumter, the United States entered an era of grave danger. By definition, the Civil War was an internal threat. But the Confederacy was also a de facto foreign state, which raised armies, issued its own currency and wrote its own constitution. The grand struggle sometimes widened Northern divisions, for example, military failures and the push for emancipation led to the rise of the “Copperheads” or peace Democrats who urged a negotiated settlement. But in a larger sense, the Confederate threat caused much of Northern opinion to coalesce around a crusade to utterly crush the Confederacy and destroy its slave system. For the first time in U.S. history, the American flag was widely draped from churches, storefronts and people’s homes. “The coldest conservatives sprang forward to the front,” said Lincoln’s secretary John Hay, “and the wildest radicals kept time with the new music.”

For many Northerners, the common struggle helped to overcome racial and other divisions. The extraordinary black commitment to the cause (10 percent of the Union army were African American) led many Northerners to reassess their views on race, and blacks made significant gains through emancipation and suffrage rights. However, those on the wrong side of the boundary drew the crusaders’ wrath—most notably the soldiers and civilians of the Confederacy. Over time, the Northern campaign evolved closer to a total war, as Union forces destroyed towns, mills and food stocks in much of the South.

Overall, the Confederate threat bonded the Northern fighters together and produced a sharper sense of collective identity. In his First Inaugural Address in 1861, Lincoln used the word “Union” twenty times and never said “nation.” Two years later, in the Gettysburg Address, he used “nation” five times and never said “Union.” The poet and historian Carl Sandburg noted that before the Civil War, people said the United States are, but afterward, they said the United States is.

When threats dissipate, domestic divisions often resurface and widen. The post–Civil War era was a time of diminishing external danger and also fraying unity within the winning coalition. During Southern Reconstruction, U.S. troops acted as peacekeepers in the former Confederacy, setting up new governments, overseeing elections and protecting the rights of the former slaves. Within a few years, much of Northern opinion saw Reconstruction as an endless quagmire. In 1871, the New York Times said the “mere mention of [Reconstruction] is almost nauseating.” The coalition to protect black rights disintegrated, the nation-building mission in the South was abandoned and white racial hegemony reemerged.

The period of security and fractiousness continued into the late nineteenth century. Historian Richard Hofstadter described a “psychic crisis” in the 1890s, triggered by the depression of 1893, the closing of the frontier, the rise of the Populist movement and the divisive election of 1896, as well as “restless aggressiveness, a desire to be assured that the power and vitality of the nation were not waning.” This crisis encouraged Americans to fight the Spanish-American War in 1898 and reclaim the unifying spirit of ’65.

The era of safety abruptly ended when the United States entered World War I in 1917. A combination of fear, anger, idealism and a sophisticated government propaganda machine produced a profound unifying effect. The Nation described a “rebirth of American patriotism,” with flags being flown across the land. Many former pacifists were swept up in the romanticism of the campaign and endorsed the war to end all war. For the holdouts who resisted the glorious crusade, the price was severe. After Sen. Robert La Follette voted against the war, his image was hanged in effigy and critics suggested he join the Reichstag.

At first glance, the 1920s seem to be an exception, where foreign threats receded but Americans were nevertheless unified, seeking what Warren Harding called “normalcy.” But the reality was far more restive. David J. Goldberg titled his history of the 1920s Discontented America. The great crusade of 1917–18 ended in military victory but not the idealistic triumph that Woodrow Wilson had promised, and disillusionment quickly set in. Americans turned inward, and the interwar years were marked by open strife between business and labor, the Ku Klux Klan and immigrants, internationalists and isolationists, progressives and conservatives, and Wets and Drys fighting over Prohibition.

During the 1940s, the threat pendulum swung again, and the United States entered an era of severe danger during World War II and the early Cold War. American security was menaced by the great powers of Germany, Japan and the Soviet Union, as well as by new technologies, including long-range bombers, missiles and nuclear weapons, which directly threatened the homeland. As would be expected, the 1940s and 1950s were a time of relative social unity. Despite hundreds of thousands of casualties, public support for World War II remained strong throughout. Americans were also willing to take the gloves off to win, and showed little concern about targeting enemy civilians through mass bombing of German and Japanese cities.

The mood of unity continued during the “Cold War consensus” of the 1950s, when there was broad agreement among Republicans and Democrats that the country was engaged in a global struggle against a Communist adversary set on world conquest. The Korean War (1950–53) proved unpopular when it bogged down into a costly stalemate, but there was no large-scale antiwar movement or serious questioning of the overall strategy of containment.

As during the Civil War, an external threat can strengthen the rights of minorities—so long as they fight for the American in-group. In 1943, Washington repealed the Chinese Exclusion Act of 1882, which prohibited almost all Chinese immigration to the United States and stopped Chinese immigrants from becoming naturalized citizens. The campaign against Nazi ideology and the alliance with China provided impetus to end an explicitly racist policy. Similarly, the Cold War was a major factor in the Supreme Court’s 1954 decision, Brown v. Board of Education, which declared racial segregation of public schools to be unconstitutional. The Justice Department filed an amicus brief in favor of desegregation that was focused solely on the negative foreign-policy effects of racial discrimination. The United States was competing with the USSR for the allegiance of newly decolonized countries: “Racial discrimination furnishes grist for the Communist propaganda mills.”

Groups associated with the enemy, however, fared far worse, including Japanese Americans who were interned during World War II and suspected Communists who were imprisoned or lost their jobs during the McCarthy era. In 1951, the Supreme Court upheld the conviction of members of the Communist Party for planning the violent overthrow of the United States, on the basis that they had read and discussed works by Karl Marx and Joseph Stalin that advocated revolution. This decision might seem to be starkly at odds with Brown v. Board of Education, because the former case diminished people’s rights, whereas the latter case expanded people’s rights. But these decisions were two sides of the same Cold War coin: aiding Americans within the in-group boundary, and targeting un-Americans outside the boundary, in order to more effectively prosecute the global contest.

By the mid-1960s, the severity of foreign threats to the United States began diminishing. In the wake of the 1962 Cuban Missile Crisis, the superpowers stabilized relations and effectively recognized each other’s legitimacy. In 1963, Washington and Moscow established a direct hotline for communication and signed the Partial Test Ban Treaty. There was tacit agreement on the ground rules of peaceful coexistence, including nonuse of nuclear weapons except as a last resort. Americans began to question the monolithic nature of Communism and see opportunities to exploit the split between Communist China and the USSR.

THE EASING of superpower relations weakened the Cold War consensus and widened political divisions, facilitating the rise of the anti–Vietnam War movement. As Cold War certainties were questioned, the in-group/out-group boundaries were blurred, creating greater discord for those inside the fence, but also new opportunities for radical critics stuck on the outside and formerly exiled from public discourse.

In the 1980s and 1990s, foreign threats to the United States declined sharply; the Cold War was over and the Soviet Union had collapsed. This happy tale had a sting because the presidencies of Bill Clinton, George W. Bush and Barack Obama were marked by heightened dissension. “We are going to do a terrible thing to you,” said the Soviet political scientist Georgy Arbatov: “We are going to take away your enemy.” The 1990s were a time of peace and prosperity—but also uncertainty and confusion about America’s place in the world. In a similar manner to the “Roaring Twenties,” the lack of an external threat meant there was no common sense of what defined the national interest. Interventions against vague or distant dangers, or in pursuit of humanitarian goals, in Somalia, Haiti, Bosnia and Kosovo, proved unpopular with the public and a Republican Congress. Trust in national institutions eroded. Politics became more sharply partisan and President Clinton was impeached by a Republican House (but acquitted by a Democratic Senate).

The United States appeared to enter an era of threats after 9/11. George W. Bush described “years of relative quiet, years of repose, years of sabbatical, and then there came a day of fire.” John Ashcroft, then attorney general, said, “A calculated, malignant, devastating evil has arisen in our world.” Terrorism can have a profound psychological impact, creating more loyalty to in-groups, and greater prejudice toward people associated with the enemy, like Muslims or immigrants. Indeed, the 9/11 attacks unified the nation, propelled Bush’s approval ratings to stratospheric heights and produced overwhelming support for a long-term global campaign against terrorism. On the evening of 9/11, members of Congress gathered on the steps of the Capitol and sang “God Bless America.” Conservatives described the struggle against radical Islam as World War IV, as they had branded the struggle against Communism as World War III. From the liberal side of the aisle, CBS anchor Dan Rather announced: “George Bush is the president. . . . Wherever he wants me to line up, just tell me where.”

But in reality the threat pendulum only shifted modestly, and the United States remained in an era of safety. The danger of a few thousand jihadists pales in comparison to historical great powers, such as Nazi Germany or the Soviet Union. The destruction of the World Trade Center proved to be an aberration rather than a harbinger of things to come, and terrorists killed only a handful of Americans in North America during the subsequent decade. For all of its psychological resonance, the threat of terrorism is simply not strong enough to unite Americans for long. As a result, the 9/11-rally effect faded. Foreign wars in Afghanistan and Iraq were not unifying crusades but divisive adventures. Public support for the missions eroded and there has been far greater self-criticism, for example, about potential U.S. war crimes such as the Abu Ghraib scandal, compared to grand struggles like World War II. Largely safeguarded from external threats, Americans looked for enemies closer to home, as partisan and other divisions widened. A host of contractors, consultants, bureaucrats, and politicians sought to take advantage of the terrorism issue to pursue their parochial interests.

IN SUMMARY, the presence or absence of external danger has always had a profound impact on American domestic politics. Eras of threat are characterized by American unity and consensus; but conformity is stifling and dissent is not tolerated. Eras of safety are characterized by the acceptance of radical and self-critical perspectives, but there is a strong sense of fractiousness as well as uncertainty about America’s role in the world. Oftentimes, outside danger is only noticed in American politics when the menace is present. But the absence of threat is also a fundamental dynamic.

Today, the United States exhibits the classic traits of an era of safety. People are restless, divided, partisan, distrustful of institutions, unsure of America’s identity and role in the world, and skeptical about the use of force. They want to focus on domestic issues, or carry out “nation building at home” as Obama put it. At the same time, there is an acceptance of diverse and even radical political views, and a willingness to question whether the country is on the right path. The Clinton versus Trump campaign is precisely what politics can look like in an era of low threats, with little agreement on foreign policy, and a bitter and even scornful tone.

What’s the solution? We should not yearn for an enemy to arise, or still less, follow the secretary of state’s advice in 1861 and deliberately create one. Eras of foreign danger threaten the lives of Americans, centralize power and undermine the rights of people associated with the danger. Oftentimes, discord is a good thing. The Cold War consensus of the 1950s might have appeared harmonious, but important social problems were swept under the rug.

Instead, presidents have to unify the country the hard way—without a powerful external danger. One answer is to rally people around a positive project like sending a man to the moon. The problem is that threats are more effective unifiers than opportunities. “The British are coming” has greater impetus than “let’s find a cure for cancer.” Indeed, in the 1960s, public support for space exploration was only lukewarm, and this backing was driven in large part by the desire to beat the Russians. Global warming might qualify as a genuine threat to the United States, but it is unlikely to bring Americans together. Psychologists have found that people are more motivated by dangers where there is a responsible agent (like the Soviet Union) rather than impersonal dangers (like global warming).

The effects of threat and safety on domestic politics are not easily overcome. A certain amount of dissension is the price of security. But presidents can dampen down antagonisms and challenge those who seek to divide Americans. At the same time, the White House can offer a positive agenda that is bold, emotionally resonant and appeals to the idealism of Americans—for example, enabling a new era of social mobility.

Americans live in a fortunate, if fractious, era. Sooner or later, they will find themselves confronting a gravely threatening evil, and enviously recall this age of security. Americans need something to fight for—before they find someone to fight against.

Dominic Tierney is associate professor of political science at Swarthmore College, a contributing editor at the Atlantic and a senior fellow at the Foreign Policy Research Institute. His most recent book is The Right Way to Lose a War: America in an Age of Unwinnable Conflicts.

Image: An F/A-18E Super Hornet aircraft on the flight deck of the aircraft carrier USS Harry S. Truman. Flickr/U.S. Department of Defense