Eighty-two percent of Americans spontaneously displayed the flag in the days just after the Sept. 11, 2001, attacks, according to a Gallup poll. Some 150 members of Congress from both parties gathered on the Capitol steps and sang “God Bless America.” Soon thereafter, a huge bipartisan majority voted to give President George W. Bush near carte blanche authority to wage a retaliatory “war on terror.”

In January 2001, 55 percent of the public described themselves as “extremely proud” to be American, according to Gallup; after what were widely understood as murderous attacks on U.S.-style freedom itself, that number surged, reaching 70 percent by June 2003.

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At the time, many thought 9/11’s galvanizing impact on the American body politic would last, that it would help close the alarming partisan fissures opened during the 1998 impeachment of Bill Clinton and the disputed 2000 presidential election. National unity would be the redeeming consequence of an otherwise baleful day.

It was a plausible hope: Throughout our history, external threats have forced the American people to put aside their differences for the sake of the common defense.

Obviously, it has not worked out that way. Gone, long gone, are the singing, flag-waving and bipartisanship of the 9/11 aftermath. In their place, we have a polarized, hate-filled political climate often, and not inaccurately, described as a “cold civil war.” In July, Gallup found that only 47 percent of the public is still “extremely proud” to be American.

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What went wrong? Many things, including the current president’s weaponization of political, cultural and racial differences. In truth, though, all of those differences pre-dated Donald Trump’s rise. His election was a symptom of decaying national consensus, not an independent cause of it.

In hindsight, we never should have expected the war on terror to unify the United States, as the two world wars or, to a large extent, the Cold War did.

Those conflicts pitted the United States against nation-states whose defeat — or, in the case of the Soviet Union, containment — could be intelligibly defined and realistically anticipated. Bright lines could be drawn between friend and foe. Rules of war could be more readily identified and upheld.

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The war that began on 9/11 and continues today, in various forms, in Afghanistan, Iraq, Syria, Yemen — even parts of Africa — was and is a protracted, shadowy conflict against irregular forces. You might call it counterinsurgency on a global scale. By its nature, such a war forces difficult and divisive policy questions on any democratic government obliged to wage it.

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Should we fire missiles from drones even when terrorists reside among civilians? Dare we use “enhanced interrogation” to question suspects? Dare we not? Should we try captives in military courts or civilian ones? Is the Islamic State part of the same enemy against which Congress authorized the use of force in 2001, or does the Constitution require fresh legal authority to fight it?

Given that the 9/11 terrorists infiltrated the United States and, in theory, the enemy could be anywhere, there are also the insidious pressures to keep out foreigners and to mount increasing surveillance domestically, which have spawned bitter debates.

At first, the war on terror did take the form of conventional warfare: The Bush administration toppled the Taliban regime in Afghanistan and, with congressional approval, launched a spinoff war against Saddam Hussein’s regime in Iraq. After quick apparent victories, though, those efforts both devolved into prolonged and costly guerrilla wars.

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Bush deserves blame for the politicized manner in which he and his team pursued the conflict, staking out extreme positions on, say, denying Geneva Convention protections to captive terror suspects, then denouncing critics as insufficiently committed to victory.

Yet divisions over the conduct of prolonged irregular warfare were bound to intensify sooner or later, as they did during the Vietnam War. By its nature, such a conflict breeds difficult policy dilemmas, and democracy, by its nature, processes such controversies through partisan conflict.

It is surely no accident that the current president and his predecessor, for all their many fundamental differences, campaigned as critics of how the war against terror was being conducted. They just stood at opposite ends of the spectrum: Barack Obama condemned the prison at Guantanamo, and Trump promised to do “worse than waterboarding” and to ban Muslims from the country.

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A war on terror, in short, offers politicians, and the public, opportunities to fight over emotionally fraught but hard-to-solve tactical dilemmas. And an endless war on terror offers them endless opportunities.