Perhaps the tensest moment in Saturday's Republican presidential debate came when Donald Trump finally said something so outrageous that the other candidates onstage and even the debate audience closed ranks against him.

Here is what Trump did: He accused George W. Bush of launching the Iraq War based on a lie:

Trump's 10-second history of the war articulated it as many Americans, who largely consider that war a mistake, now understand it. And, indeed, Bush did justify the war as a quest for Iraqi weapons of mass destruction, which turned out not to exist.

The other Republican candidates, who have had this fight with Trump before, did not defend the war as their party has in the past, but rather offered the party's standard line of the moment, which is that Bush had been innocently misled by "faulty intelligence."

But neither version of history is really correct. The US primarily invaded Iraq not because of lies or because of bad intelligence, though both featured. In fact, it invaded because of an ideology.

A movement of high-minded ideologues had, throughout the 1990s, become obsessed with deposing Saddam Hussein. When they assumed positions of power under Bush in 2001, they did not seek to trick America into that war, but rather tricked themselves. In 9/11, and in fragments of intelligence that more objective minds would have rejected, they could see only validation for their abstract and untested theories about the world — theories whose inevitable and obvious conclusion was an American invasion of Iraq.

This is perhaps not as satisfying as the "Bush lied, people died" bumper sticker history that has since taken hold on much of the left and elements of the Tea Party right. Nor is it as convenient as the Republican establishment's polite fiction that Bush was misled by "faulty intelligence."

If the problem were merely that Bush lied, then the solution would be straightforward: Check the administration's facts. But how do you fact-check an ideology, particularly when that ideology is partially concealed from the public view? How do you guard against that ideology, which still dominates much of the GOP, and some of whose ideas are shared by more hawkish Democrats, from leading us astray again?

The moment at Saturday's debate should highlight the degree to which many Americans, from voters right up to presidential candidates, still misunderstand — and failed to learn from — the story of how America came to expend 4,500 of its citizens' lives in a war that would kill well over 100,000 Iraqis, destroy an entire nation, and help send the Middle East spiraling into chaos.

Why did the United States invade Iraq?

To understand the American decision to invade Iraq, and to learn the lessons of that mistake, one must begin not with George W. Bush's claims of Iraqi WMDs or with the 9/11 attacks, but rather with a series of initially obscure ideological debates on elements of the American right.

Those debates, which played out throughout the 1990s, had their roots in disagreements within the Republican Party over American power — and in the evolution of a right-leaning but surprisingly heterodox intellectual movement known as neoconservatism.

Neoconservatism, which had been around for decades, mixed humanitarian impulses with an almost messianic faith in the transformative virtue of American military force, as well as a deep fear of an outside world seen as threatening and morally compromised.

This ideology stated that authoritarian states were inherently destabilizing and dangerous; that it was both a moral good and a strategic necessity for America to replace those dictatorships with democracy — and to dominate the world as the unquestioned moral and military leader.

Neoconservatism's proponents, for strategic as well as political reasons, would develop an obsession with Saddam Hussein's Iraq. That obsession would, by the end of the decade, congeal into a policy, explicitly stated: regime change.

Their case was always grandly ideological, rooted in highly abstract and untested theories about the nature of the world and America's rightful place in it. Their beliefs were so deeply held that when 9/11 shook the foundations of American foreign policy, they were able to see only validation of their worldview, including their belief in the urgent need to bring democracy to Iraq.

It was this ideological conviction, more than any piece of intelligence or lie told about it, that primarily led America into Iraq. Weapons of mass destruction were the stated justification, but they were never the real reason, nor was bad intelligence.

The lesson of the Iraq mistake is not the dangers of lying or of anything as narrow as faulty intelligence, but rather of sweeping ideologies and ambitions that can take on a momentum all their own.

That particular ideology, neoconservatism, remains a major force in the Republican Party, and a number of its tenets are held by some Democrats as well. Its mandate for war, and its faith in the power of American military force, still animates that ideology, particularly toward the Middle East.

It is remarkable and alarming that more than a decade and thousands of lives later, neither Republicans nor Americans more broadly have fully confronted how that ideology developed to lead us into a catastrophic war — and the dangers that it, or any other blindly fervent ideology on the right or the left, could still pose.

The radical ideas that led to the neoconservative obsession with Iraq

The story of neoconservatism's evolution in the 1990s begins and ends with Iraq, but at its start it was a disagreement among Republicans.

In late 1990, Saddam Hussein's Iraq invaded the oil-rich neighboring kingdom of Kuwait, and a few months later President George H.W. Bush led a brief military intervention to expel Saddam.

But where many Americans saw a rousing success, and the start of a decade that they would experience as overwhelmingly peaceful, a dissident faction of Republicans in and outside of the administration experienced it as a formative moment of national disgrace.

As the American-led mission wound down, the elder Bush urged Iraqis to rise up. But Bush had stopped the war short of destroying Saddam's Republican Guard or his helicopter units, which were able to quickly crush the short-lived Iraqi uprising.

Some administration officials, particularly then-Under Secretary of Defense Paul Wolfowitz, argued that the US should intervene against Saddam's crackdown — if not to aid in regime change, then at least to stop the slaughter.

Wolfowitz "wanted to finish Saddam's regime, and not only did he want to finish it, he believed there was a strong basis for doing so," Richard Perle, another major neoconservative figure, told the journalist George Packer for his book The Assassins' Gate.

Wolfowitz, an idealist and humanitarian, had long believed in America's responsibility to promote democracy abroad. In the mid-1980s, as Ronald Reagan's assistant secretary of state for East Asia, Wolfowitz successfully pushed for the US to abandon Filipino dictator Ferdinand Marcos, who, though a reliable anti-communist, was violent and corrupt.

For Wolfowitz and other neoconservatives in the elder Bush administration, the 1991 Gulf War embodied of everything that was morally wrong — and indeed dangerous — with America's practice of tolerating dictators.

Throughout the 1990s, Saddam Hussein only became more defiant and disobedient, ignoring United Nations mandates on weapons inspections and issuing increasingly anti-American rhetoric. While many Middle East analysts suspected Saddam's actions were primarily designed to help him save face at home after his humiliating 1991 defeat against the Americans, neoconservatives saw not just American humiliation but alarming evidence of American decline.

This played into a growing school of thought among the dissident Republicans, which went far beyond Iraq. It said that America had a special responsibility to spread democracy for the betterment of humanity, that Republicans had forgotten the world-changing idealism of Ronald Reagan, and that the end of the Cold War was not an excuse for America to retreat from its military adventurism but rather the moment when it was needed most.

A historian and scholar named Robert Kagan helped lead this charge. He argued that America's unilateral assertion of power — the mere fact of American military action — was not just strategically but morally necessary. It would spread democracy and thus human rights, but also deter rogue states and thus promote peace.

In 1996, Kagan co-authored, along with Weekly Standard editor Bill Kristol, a seminal essay in Foreign Affairs calling on America to bring about an era of "global benevolent hegemony."

They predicted that the world would welcome American military dominance as a force for stability and for the promotion of values such as democracy and human rights. In this view, nearly any expression of American military dominance was an act of moral good, whereas the absence of US dominance would invite chaos and, ultimately, threats against the US.

The neoconservatives' attention would inevitably return, over and over, to Iraq and to the anti-American dictator who had wrongly escaped justice. Iraq was a perfect example of their criticisms of Democrats and Republicans alike, its defiance a seemingly undeniable argument for their worldview.

Building the case for war

In 1997, the year after their Foreign Affairs essay, Kagan and Kristol helped found a group called the Project for a New American Century, meant to instill these foreign policy ambitions in a Republican Party that had tilted away from Reagan-style idealism.

PNAC included in its members Wolfowitz and Perle, as well as other senior Reagan administration officials and neoconservatives such as Elliott Abrams, James Woolsey, and Donald Rumsfeld. From the start, it made Iraq its central issue.

In January 1998, PNAC published an open letter to the Clinton administration warning that "we may soon face a threat in the Middle East more serious than any we have known since the end of the Cold War." It urged a new strategy that "should aim, above all, at the removal of Saddam Hussein’s regime from power."

Partly this was specific to Iraq. The world was generally pliant to American will in the 1990s, but the defiantly anti-American Iraq stood out as a glaring exception; neoconservatives simply had few other examples to justify their view of a dangerous world that had to be subjugated by American power.

Perhaps just as importantly, Iraq was seen in Washington as a policy failure for Bill Clinton — tempting many Republicans, whether they were particularly invested in neoconservatism or not, to take hard-line positions from which to attack him.

But more than that, this was about using Iraq as a proving ground for the neoconservatives' larger and more ideological mission.

"They saw Iraq as the test case for their ideals about American power and world leadership," Packer writes. "Iraq represented the worst failure of the nineties and the first opportunity of the new American century."

As it happened, PNAC and its allies had an unprecedented opening to harden their radical proposal into mainstream Washington consensus.

In 1998 came the Monica Lewinsky scandal, in which congressional Republicans, sensing Clinton's political weakness, sought opportunities to both embarrass him on other fronts and win concessions he might have otherwise resisted. Iraq gave them both: That October, seizing on PNAC's call for regime change, congressional Republicans passed the Iraq Liberation Act, which stated that regime change was US policy.

Clinton caved to the pressure, signing the Iraq Liberation Act and thus announcing to Saddam Hussein, and to the world, that America was bent on his removal. Saddam, in retaliation, expelled UN weapons inspectors that same day.

These two acts would prove crucial in laying the groundwork for the US invasion five years later. In Washington, regime change had suddenly and with little thought become a comfortably bipartisan policy position. And the George W. Bush administration would later argue that Saddam had expelled the inspectors not as political retaliation, but rather to restart his 1980s chemical and biological weapons programs.

In the final year of Clinton's presidency, Kristol and Kagan co-edited a book of essays titled Present Dangers, meant to argue for a new era of neoconservative Republican foreign policy. It included an essay by Richard Perle that argued the US should not just promote an Iraqi uprising but also provide US ground troops to assist them. Perle also urged installing in Saddam's place an exile group known as the Iraqi National Congress, which was headed by Ahmed Chalabi — the very man the US would try to install three years later.

A few months later, Texas Gov. George W. Bush became president. Moved by neoconservatism's idealistic faith in democracy and perhaps sympathetic to its fixation on Iraq — Saddam had attempted to assassinate Bush's father — Bush filled several top positions with members of PNAC and other neoconservative adherents, including Rumsfeld as defense secretary and Wolfowitz as deputy secretary of defense. Richard Perle chaired the Pentagon's defense policy advisory board.

What 9/11 really had to do with the Iraq War

Despite longstanding conspiracy theory to the contrary, it is not the case that Bush came into office secretly plotting to invade Iraq or that he seized on the 9/11 attacks as cynical justification. While there is a line between the attacks and the invasion of Iraq, that line is not as direct as many Americans might think.

The attacks left Bush, a foreign policy neophyte, adrift. He had little experience with the Middle East or the complex social and political forces that had culminated, seemingly out of nowhere, in the deaths of some 3,000 Americans. He grasped for an answer; the neoconservatives in his administration just happened to have one ready.

Since long before 9/11, these officials had argued that terrorism like that of al-Qaeda had to be understood as a symptom of the Middle East's real problems as they saw it: an absence of democracy and of American-dominated "benevolent hegemony."

This worldview did not necessarily require that Saddam Hussein had been behind the 9/11 attacks or that he had sheltered Osama bin Laden. Nonetheless, the neoconservatives, so steeped in abstract ideological convictions that put Saddam at the center of the Middle East's problems, were unable to resist the temptation to see the 9/11 attacks as validating their grand theories about the world.

And those theories inevitably culminated, as they always had, in the need for America to go to war with Iraq.

On 9/11 itself, Packer recounts in his book, "Within minutes of fleeing his office at the devastated Pentagon, Wolfowitz told aides that he suspected Iraqi involvement in the attacks."

On September 12, 2001, as rescue workers still swarmed the downed Twin Towers, Bush asked his counterterrorism team to investigate Iraqi links. "See if Saddam did this. See if he's linked in any way. ... I want to know any shred," he said, according to then-counterterrorism chief Richard A. Clarke's recollection to Packer.

On September 15, at a high-level Camp David meeting to discuss the US response to the attacks, Wolfowitz repeatedly raised Saddam Hussein as not just a possible link but the most important target for retaliation.

On September 17, according to Packer's account, Bush told his war council, "I believe Iraq was involved."

In subsequent months, the Bush administration would gesture at a case for Iraqi involvement in 9/11, but would ultimately settle on a very different argument that Saddam possessed WMD programs that threatened the US.

Bush's flexibility in how he justified the war was telling. It was not any particular issue, whether terrorism or WMDs, that prompted the war; rather, it was always about ideological convictions. Those convictions took on a momentum of their own.

The administration's neoconservatives argued not just for possible links between Saddam and Osama bin Laden, but that al-Qaeda was an outgrowth of the Middle East's larger problems as they had long identified them. Toppling Saddam would not just solve these root problems — it would transform the Middle East for the better, and begin an era of welcomed American dominance over the region.

These arguments relied increasingly on a small circle of Middle East scholars such as Fouad Ajami, whose 1998 book Dream Palace of the Arabs had rooted the region's problems in a self-perpetuating social and political rot. Only a major jolt could end the cycle and awaken the once-proud Arabs. This jolt, Ajami argued, would be best delivered by an American invasion to topple Saddam and "liberate" Iraqis with democracy — thus surely inspiring a regional awakening.

By that December, long before the Bush administration would produce any of the so-called smoking guns proving Iraqi WMDs, it had already begun preparing to sell the public on a war with Iraq. David Frum, the Bush-era speechwriter who would later coin the term "axis of evil," described this moment in his memoir, The Right Man:

Frum clarifies that other speechwriters were working on alternate drafts that were to be less "hawkish"; his assignment, he believes, did not indicate that the administration was yet dead set on war.

But Frum's anecdote, like so many others from that time, shows the building momentum, within the administration, for war — a momentum, propelled by ideological conviction, that would ultimately overtake reason and critical thinking in the White House.

In March 2002, Bush dropped into a meeting between National Security Adviser Condoleezza Rice and three senators to tell them, "Fuck Saddam. We're taking him out."

That June, Richard Haass, the State Department director of policy planning, visited Rice's office for their regular meeting. When he raised the State Department's misgivings about the "bureaucratic chatter" of a possible war, Rice cut him off.

"Save your breath," she told him. "The president has already made up his mind."

"It was an accretion, a tipping point," Haass told Packer, recounting the incident. "A decision was not made — a decision happened and you can't say when or how."

How the Bush administration fooled even itself

The neoconservative ideological convictions — a preoccupation with Saddam Hussein, a radical ambition to remake the Middle East from within, an almost blind faith in American military power as a force for positive transformation — led them to desire a war with Iraq as the solution to not just terrorism but a litany of problems, and to see validation for that desire even in the obviously flawed intelligence that would be their justification.

The White House inserted itself directly into an intelligence dissemination and vetting process that is typically handled by the agencies themselves. After 9/11, Bush and Vice President Dick Cheney instituted a new system known as "Top Secret Codeword/Threat Matrix," under which they demanded to personally review raw intelligence.

"The mistake was not to have proper analysis of the intelligence before giving to the president," Roger Cressey, who served in Bush's National Security Council, told Jane Mayer for her book The Dark Side. "There was no filter. Most of it was garbage. None of it had been corroborated or screened. But it went directly to the president and his advisers, who are not intelligence experts. That's when mistakes got made."

In the months after the attacks, US intelligence agencies came under heavy pressure to investigate the administration's suspicions of links between Saddam Hussein and 9/11, or of ongoing Iraqi WMD programs.

It does not appear that the administration encouraged them to lie, but rather that deep-rooted biases led top officials to dismiss the mountains of intelligence that undercut their theories and to favor deeply problematic intelligence that supported it.

In 2001, for example, a man named Ibn al-Sheikh al-Libi, whom the US had picked up in Afghanistan and then shipped to Egypt to be tortured, claimed that Saddam had provided al-Qaeda with chemical and biological weapons training. The Defense Intelligence Agency warned that Libi's information could not be trusted. But Bush treated it as credible, and repeated Libi's claim as established fact in his case for war.

The US also relied heavily on claims by an Iraqi exile living in Germany named Rafid Ahmed Alwan, code-named "curveball," who claimed to have direct knowledge of secret Iraqi WMD programs. Though both German and UK intelligence said Alwan was unstable and his information unreliable, the US embraced his claims, which provided the basis of much of its case for war.

Years later, Alwan admitted he had made it all up to help instigate the American invasion of Iraq. But the White House believed him for the simple reason that it badly wanted to.

Within months, the momentum for war within the administration had overtaken the normal processes of decision-making — and certainly had overtaken the public case for war.

By all appearances, administration officials believed their allegations of Iraqi WMDs were true and that this was indeed sufficient justification. Why else would the US launch a desperate, high-profile search for WMDs after invading — which only ended up drawing more attention to how false those allegations had been?

Rather, they had deceived themselves into seeing half-baked intelligence as affirming their desire for war, and then had sold this to the American people as their casus belli, when in fact it was secondary to their more high-minded and ideological mission that would have been too difficult to explain. That, more than overstating intelligence on WMDs, was the really egregious lie.

The lie bigger than WMDs: claiming the war was because of WMDs

"We know they have weapons of mass destruction. We know they have active programs. There isn't any debate about it," Rumsfeld said in September 2002.

"Saddam Hussein still has chemical and biological weapons, and is increasing his capabilities to make more. And he is moving ever closer to developing a nuclear weapon," Bush said the next month, warning that Saddam would "threaten America and the world with horrible poisons, and diseases, and gases, and atomic weapons."

Then-National Security Adviser Condoleezza Rice claimed that Saddam was running a clandestine nuclear program that was only "six months from a crude nuclear device."

In fact, none of this was true. Iraq had discontinued its chemical and biological weapons programs in the 1980s. A 1998 US-led bombing campaign had destroyed much of the remains.

But even if Bush's allegations had been true, they would not have accurately described his administration's real reasons for invading Iraq. The neoconservative mission of upending a tyrant and bringing democracy to the Middle East was mentioned only as a secondary benefit, or deployed as a later justification when no WMDs materialized.

This was, in part, how the Bush administration backed itself into such shoddy intelligence — shutting down Iraqi WMDs was never really the point, so Bush officials had little reason to fully vet the intelligence suggesting those programs were already gone.

At the same time, in keeping their actual reasons for war from the public, the Bush administration lost the opportunity for those reasons to be openly debated, at which point more grounded Middle East or military scholars might have revealed them as dangerously misguided.

America needs to finally confront the lessons of Iraq — before we repeat them

As Donald Trump's stunt showed, America's public debate over Iraq, now 13 years later, still turns largely on Bush's claims and their truth. But even if Saddam had turned out to possess weapons of mass destruction, if Bush had been right, what would it really change?

The war would still have cost some 4,500 American lives and well over 100,000 Iraqi lives. It would still have destabilized Iraq, opened up the country for violent extremism, and contributed directly to the rise of ISIS. And it would still have been launched in pursuit of an ideological mission that turned out to be dangerously misguided.

Abstract and radical neoconservative ideas that had developed during the Clinton years, bouncing around a tiny echo chamber of like-minded idealists who had little desire to challenge one another, had suddenly and with no real public debate become the basis of a war that would quickly cost many thousands of lives.

But those ideas are still very much a part of America's foreign policy discourse, and some day, even as soon as this January, their adherents could return to the White House.

Americans have rightly litigated the question of Bush's honesty on WMDs. But we have still not interrogated the deeper force behind the catastrophic war: the radical convictions of a neoconservative ideology that remains central to the Republican Party's foreign policy — particular among establishment-backed presidential candidates such as Marco Rubio and Jeb Bush.

These candidates, in how they discuss hostile nations such as Iran, Russia, and Syria, do not sound so different from the neoconservatives of the 1990s. You hear this in their belief in the power and virtue of unilateral American force, in the need to express hegemonic American dominance over the Middle East, and in the apparently earnest fear that any challenge to American power, no matter how slight, is just the start of a potentially global unraveling.

You see it in Marco Rubio's highly ideological but analytically groundless belief that dismantling the Iran nuclear deal and adopting a policy of maximal belligerence toward Tehran would advance freedom and peace in the Middle East.

This is not to say that neoconservative candidates are secretly plotting, or would necessarily execute, another war in the Middle East — although it is concerning to see them so focused on Iran as an implacable and grave threat that can only be addressed by subjugating the regime or bringing about its downfall.

It is concerning to see Rubio advocating forceful regime change in Syria and hiring a foreign policy adviser who advocates it in Iran, all along similar high-minded ideological lines as the neoconservative obsession with Iraq 20 years ago. It is worrying to hear hawks like Sen. Tom Cotton, embraced by neoconservative luminaries, explicitly advocate that the US abandon the nuclear deal to instead force regime change or even launch military strikes.

To be clear, the ideas of neoconservatism are not all exclusive to the Republican Party; Democrats such as Hillary Clinton and Samantha Power have pursued some, though far from all, similarly high-minded policies, particularly a belief in humanitarian interventions. (Indeed, Clinton voted for the Iraq War.) And many Republicans do oppose neoconservatism, instead advocating a return to the hard-nosed realism of George H.W. Bush.

The lesson is not that neoconservatism should be a disqualification from the presidency. Indeed, the ideology has made important and undervalued contributions to American foreign policy, such as its focus on human rights and its warning that supporting friendly dictatorships is both morally wrong and, in the long term, strategically unviable.

But these ideas, like neoconservatives' more dangerous faith in the transformative power of American military force, deserve to be evaluated and then either embraced or rejected on their merits.

In the Iraq War, we had the purest possible test of many of this ideology's core beliefs about the inherent virtue of American military power, about the supposedly transformative power of regime change, and about the supposed demand for American hegemony.

These ideas all proved not just false but disastrously so. We have not taken those lessons into account, preferring instead to litigate the narrower and politically easier question of Bush's personal honesty.

The lesson, which extends to both parties, is that a potential president's ideological views are just as important to examine and vet as are his or her policy proposals; that the line between obscure policy journals and American military action can be much shorter than we'd like to think.

That is true of any ideology, but it is especially true of neoconservatism, which we have still not chosen to vet, remarkably, even after we invested billions of dollars and thousands of lives in testing it directly in Iraq, to results apparently so damning we have still not fully absorbed them.