For years, the United States’ political dysfunction has been attributed to large-scale social and economic changes. Experts point to the failures of globalization, rising inequality, reduced social mobility, or the long aftermath of the “China shock” more than two decades ago to explain today’s extreme polarization, collapsing trust in institutions, democratic erosion, immigration backlash, and struggling middle class. Yet there is another widely overlooked factor that is equally central to understanding the United States’ current predicament: the war on terror.

Typically, when policymakers and analysts speak of the global war on terror—the defining fact of international affairs for two decades—they discuss the damage done to terrorist groups, the plots that have been thwarted, the rise and fall of jihadi ideology, the many intelligence failures, and, perhaps above all, the two forever wars in Afghanistan and Iraq. These are the scorecards of contemporary counterterrorism. But left out of this framing are the profound effects of this vast war on terror on domestic politics and society.

The human statistics alone tell an appalling story: more than 7,000 U.S. troops were killed in action, 53,000 were wounded, and well over 30,000 took their own lives—a staggering number. According to Brown University’s Costs of War Project, these conflicts added $8 trillion to the U.S. national debt, with trillions more projected in the future for veterans’ health care, including for the half million GIs with traumatic brain injuries and posttraumatic stress disorder.

Yet even these numbers fail to show the myriad ways in which two decades of war transformed the United States. For years, the fixation on Islamist terror shaped a culture drenched in fear, as voices from politics, the media, civil society, and a swelling counterterrorism-industrial complex insisted that the country faced an existential threat. Amid this climate, the government was given broad new powers to root out and prosecute potential jihadi activity wherever it might be, and the widespread assumption that there might be another terrorist attack began to reshape American society itself. Rather than bringing the country together behind a shared—and narrowly defined—enemy, however, the result has been a dramatic decline in social cohesion. It has contributed to the demise of political bipartisanship, already weakened in previous decades. At the same time, the war on terror fueled an alarming rise in xenophobia and anti-immigrant sentiment and discredited the country’s political and foreign policy elites.

Above all has been the uncontrolled expansion of executive power. This impetus did not begin with the war on terror; generations of American leaders and scholars have sought to increase presidential power, drawing up mechanisms that could sideline civil liberties and intertwine military and police activities. But after 9/11, these contingencies were used to give the executive branch sweeping new authority in the name of domestic security. And today, an authoritarian president is using these powers to target his political enemies. The United States, then, is overdue for a reckoning with the political legacy of the war on terror.

THE POWER OF NIGHTMARES

Anxiety about extended war has preoccupied American officials throughout U.S. history. In the first years after the country’s independence, James Madison, who would become the country’s fourth president, observed that “no nation can preserve its freedom in the midst of continual war.” More than a century and a half later, U.S. President Dwight Eisenhower sounded the alarm about “the acquisition of unwarranted influence” by what he famously called “the military-industrial complex.” As Eisenhower forewarned, “the potential for the disastrous rise of misplaced power exists and will persist.” For many of the decades since, much of that apprehension has been focused on the concentration of power in government and in the broader machinery of war. Yet during the war on terror, parts of society mobilized in parallel with the state and sought to cast a much wider net against a class of perceived enemies.

Terrorist acts are typically carried out by secret actors operating within a country’s borders, and 9/11 was the ultimate case in point. The attacks on the Twin Towers and the Pentagon—the first on U.S. soil since World War II—were carried out by foreign nationals inside the country. They were motivated by the al-Qaeda leader Osama bin Laden’s particular brand of jihadi ideology, which held that major attacks on the U.S. homeland would lead Washington to withdraw support for the apostate rulers of the Muslim world. These themes—which also carried notions of proportional revenge for the number of Muslims previously killed by the United States—and the attacks themselves gave license to Islamophobes throughout American society, including in the political world and policy establishment. The most influential came from the evangelical churches: to their huge audiences, leading televangelists such as Franklin Graham, John Hagee, and Robert Jeffress described Islam as “satanic” and a “heresy from the pit of hell” and its followers as a cult of pedophiles.

President George W. Bush sought to tamp down the hatred by insisting that the war on terror was not a war against Islam. But for all his protestations, many saw the war on terror as a crusade—a word that Bush himself used early on until he was told how poorly that played in the Islamic world. His message was further belied by such government actions as the detention of at least 1,200 Muslim men—many of whom were held on immigration charges but treated as high-security risks—and the severe curtailment of Muslim civil liberties by the FBI and other law enforcement in the name of addressing the terrorist threat. The fact that the 9/11 hijackers had been sustained by an efficient Muslim support network within the United States no doubt contributed to these measures, as did reasonable concerns about the potential for online radicalization.

Trump blurs the lines between Islamophobia and anti-immigrant sentiment.

In the decades since, the persistence of fear-mongering and the growth of anti-Muslim sentiment and hate crimes has been hard to miss. When the Obama administration sought to transfer foreign detainees captured in the war on terror from Guantánamo to facilities in the United States, Republicans depicted them as superhuman terrorists that no U.S. prison could safely secure. The plan was nixed. In 2014, when it was revealed that a jihadi’s laptop found in an Islamic State (ISIS) hideout in Idlib, Syria, contained some basic information on bioweapons, members of the flourishing counterterrorism commentariat claimed that terrorists planned to cull the African Ebola virus from victims’ corpses and spray it widely in American urban centers. Candidates for high office found the terrorist threat to be the gift that never ceased giving. Political scientists at Dartmouth, where both of us have taught, recall hearing former New Jersey Governor Chris Christie, a candidate for the Republican presidential nomination in 2016, tell elderly women in a remote New Hampshire village that jihadis might take them hostage in their homes if they didn’t vote for the tough-on-terror candidate.

Claims such as these were contradicted by the actual state of the threat. By 2016, it had been several years since U.S. Navy SEALS had killed bin........

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