In the interviews, more than 400 insiders offered unrestrained criticism of what went wrong in Afghanistan and how the United States became mired in nearly two decades of warfare.

With a bluntness rarely expressed in public, the interviews lay bare pent-up complaints, frustrations and confessions, along with second-guessing and backbiting.

“We were devoid of a fundamental understanding of Afghanistan — we didn’t know what we were doing,” Douglas Lute, a three-star Army general who served as the White House’s Afghan war czar during the Bush and Obama administrations, told government interviewers in 2015. He added: “What are we trying to do here? We didn’t have the foggiest notion of what we were undertaking.”

“If the American people knew the magnitude of this dysfunction . . . 2,400 lives lost,” Lute added, blaming the deaths of U.S. military personnel on bureaucratic breakdowns among Congress, the Pentagon and the State Department. “Who will say this was in vain?”

Since 2001, more than 775,000 U.S. troops have deployed to Afghanistan, many repeatedly. Of those, 2,300 died there and 20,589 were wounded in action, according to Defense Department figures.

The interviews, through an extensive array of voices, bring into sharp relief the core failings of the war that persist to this day. They underscore how three presidents — George W. Bush, Barack Obama and Donald Trump — and their military commanders have been unable to deliver on their promises to prevail in Afghanistan.

With most speaking on the assumption that their remarks would not become public, U.S. officials acknowledged that their warfighting strategies were fatally flawed and that Washington wasted enormous sums of money trying to remake Afghanistan into a modern nation.

The interviews also highlight the U.S. government’s botched attempts to curtail runaway corruption, build a competent Afghan army and police force, and put a dent in Afghanistan’s thriving opium trade.

The U.S. government has not carried out a comprehensive accounting of how much it has spent on the war in Afghanistan, but the costs are staggering.

Since 2001, the Defense Department, State Department and U.S. Agency for International Development have spent or appropriated between $934 billion and $978 billion, according to an inflation-adjusted estimate calculated by Neta Crawford, a political science professor and co-director of the Costs of War Project at Brown University.

Those figures do not include money spent by other agencies such as the CIA and the Department of Veterans Affairs, which is responsible for medical care for wounded veterans.

“What did we get for this $1 trillion effort? Was it worth $1 trillion?” Jeffrey Eggers, a retired Navy SEAL and White House staffer for Bush and Obama, told government interviewers. He added, “After the killing of Osama bin Laden, I said that Osama was probably laughing in his watery grave considering how much we have spent on Afghanistan.”

The documents also contradict a long chorus of public statements from U.S. presidents, military commanders and diplomats who assured Americans year after year that they were making progress in Afghanistan and the war was worth fighting.

Several of those interviewed described explicit and sustained efforts by the U.S. government to deliberately mislead the public. They said it was common at military headquarters in Kabul — and at the White House — to distort statistics to make it appear the United States was winning the war when that was not the case.

“Every data point was altered to present the best picture possible,” Bob Crowley, an Army colonel who served as a senior counterinsurgency adviser to U.S. military commanders in 2013 and 2014, told government interviewers. “Surveys, for instance, were totally unreliable but reinforced that everything we were doing was right and we became a self-licking ice cream cone.”

John Sopko, the head of the federal agency that conducted the interviews, acknowledged to The Post that the documents show “the American people have constantly been lied to.”

The interviews are the byproduct of a project led by Sopko’s agency, the Office of the Special Inspector General for Afghanistan Reconstruction. Known as SIGAR, the agency was created by Congress in 2008 to investigate waste and fraud in the war zone.

In 2014, at Sopko’s direction, SIGAR departed from its usual mission of performing audits and launched a side venture. Titled “Lessons Learned,” the $11 million project was meant to diagnose policy failures in Afghanistan so the United States would not repeat the mistakes the next time it invaded a country or tried to rebuild a shattered one.

The Lessons Learned staff interviewed more than 600 people with firsthand experience in the war. Most were Americans, but SIGAR analysts also traveled to London, Brussels and Berlin to interview NATO allies. In addition, they interviewed about 20 Afghan officials, discussing reconstruction and development programs.

Drawing partly on the interviews, as well as other government records and statistics, SIGAR has published seven Lessons Learned reports since 2016 that highlight problems in Afghanistan and recommend changes to stabilize the country.

But the reports, written in dense bureaucratic prose and focused on an alphabet soup of government initiatives, left out the harshest and most frank criticisms from the interviews.

“We found the stabilization strategy and the programs used to achieve it were not properly tailored to the Afghan context, and successes in stabilizing Afghan districts rarely lasted longer than the physical presence of coalition troops and civilians,” read the introduction to one report released in May 2018.

The reports also omitted the names of more than 90 percent of the people who were interviewed for the project. While a few officials agreed to speak on the record to SIGAR, the agency said it promised anonymity to everyone else it interviewed to avoid controversy over politically sensitive matters.

Under the Freedom of Information Act, The Post began seeking Lessons Learned interview records in August 2016. SIGAR refused, arguing that the documents were privileged and that the public had no right to see them.

The Post had to sue SIGAR in federal court — twice — to compel it to release the documents.

The agency eventually disclosed more than 2,000 pages of unpublished notes and transcripts from 428 of the interviews, as well as several audio recordings.

The documents identify 62 of the people who were interviewed, but SIGAR blacked out the names of 366 others. In legal briefs, the agency contended that those individuals should be seen as whistleblowers and informants who might face humiliation, harassment, retaliation or physical harm if their names became public.

By cross-referencing dates and other details from the documents, The Post independently identified 33 other people who were interviewed, including several former ambassadors, generals and White House officials.

The Post has asked a federal judge to force SIGAR to disclose the names of everyone else interviewed, arguing that the public has a right to know which officials criticized the war and asserted that the government had misled the American people. The Post also argued the officials were not whistleblowers or informants, because they were not interviewed as part of an investigation.

A decision by Judge Amy Berman Jackson of the U.S. District Court in Washington has been pending since late September.

The Post is publishing the documents now, instead of waiting for a final ruling, to inform the public while the Trump administration is negotiating with the Taliban and considering whether to withdraw the 13,000 U.S. troops who remain in Afghanistan.

The Post attempted to contact for comment everyone whom it was able to identify as having given an interview to SIGAR. Their responses are compiled in a separate article.

Sopko, the inspector general, told The Post that he did not suppress the blistering criticisms and doubts about the war that officials raised in the Lessons Learned interviews. He said it took his office three years to release the records because he has a small staff and because other federal agencies had to review the documents to prevent government secrets from being disclosed.

“We didn’t sit on it,” he said. “We’re firm believers in openness and transparency, but we’ve got to follow the law. . . . I think of any inspector general, I’ve probably been the most forthcoming on information.”

The interview records are raw and unedited, and SIGAR’s Lessons Learned staff did not stitch them into a unified narrative. But they are packed with tough judgments from people who shaped or carried out U.S. policy in Afghanistan.

“We don’t invade poor countries to make them rich,” James Dobbins, a former senior U.S. diplomat who served as a special envoy to Afghanistan under Bush and Obama, told government interviewers. “We don’t invade authoritarian countries to make them democratic. We invade violent countries to make them peaceful and we clearly failed in Afghanistan.”

To augment the Lessons Learned interviews, The Post obtained hundreds of pages of previously classified memos about the Afghan war that were dictated by Defense Secretary Donald H. Rumsfeld between 2001 and 2006.

Dubbed “snowflakes” by Rumsfeld and his staff, the memos are brief instructions or comments that the Pentagon boss dictated to his underlings, often several times a day.

Rumsfeld made a select number of his snowflakes public in 2011, posting them online in conjunction with his memoir, “Known and Unknown.” But most of his snowflake collection — an estimated 59,000 pages — remained secret.

In 2017, in response to a FOIA lawsuit filed by the National Security Archive, a nonprofit research institute based at George Washington University, the Defense Department began reviewing and releasing the remainder of Rumsfeld’s snowflakes on a rolling basis. The Archive shared them with The Post.

Together, the SIGAR interviews and the Rumsfeld memos pertaining to Afghanistan constitute a secret history of the war and an unsparing appraisal of 18 years of conflict.

Worded in Rumsfeld’s brusque style, many of the snowflakes foreshadow problems that continue to haunt the U.S. military more than a decade later.

“I may be impatient. In fact I know I’m a bit impatient,” Rumsfeld wrote in one memo to several generals and senior aides. “We are never going to get the U.S. military out of Afghanistan unless we take care to see that there is something going on that will provide the stability that will be necessary for us to leave.”

“Help!” he wrote.

The memo was dated April 17, 2002 — six months after the war started.

With their forthright descriptions of how the United States became stuck in a faraway war, as well as the government’s determination to conceal them from the public, the cache of Lessons Learned interviews broadly resembles the Pentagon Papers, the Defense Department’s top-secret history of the Vietnam War.

When they were leaked in 1971, the Pentagon Papers caused a sensation by revealing the government had long misled the public about how the United States came to be embroiled in Vietnam.

Bound into 47 volumes, the 7,000-page study was based entirely on internal government documents — diplomatic cables, decision-making memos, intelligence reports. To preserve secrecy, Defense Secretary Robert McNamara issued an order prohibiting the authors from interviewing anyone.

SIGAR’s Lessons Learned project faced no such restrictions. Staffers carried out the interviews between 2014 and 2018, mostly with officials who served during the Bush and Obama years.

About 30 of the interview records are transcribed, word-for-word accounts. The rest are typed summaries of conversations: pages of notes and quotes from people with different vantage points in the conflict, from provincial outposts to the highest circles of power.

Some of the interviews are inexplicably short. The interview record with John Allen, the Marine general who commanded U.S. and NATO forces in Afghanistan from 2011 to 2013, consists of five paragraphs.

In contrast, other influential figures, including former U.S. ambassador Ryan Crocker, sat for two interviews that yielded 95 transcribed pages.

Unlike the Pentagon Papers, none of the Lessons Learned documents were originally classified as a government secret. Once The Post pushed to make them public, however, other federal agencies intervened and classified some material after the fact.

The State Department, for instance, asserted that releasing portions of certain interviews could jeopardize negotiations with the Taliban to end the war. The Defense Department and Drug Enforcement Administration also classified some interview excerpts.

The Lessons Learned interviews contain few revelations about military operations. But running throughout are torrents of criticism that refute the official narrative of the war, from its earliest days through the start of the Trump administration.

At the outset, for instance, the U.S. invasion of Afghanistan had a clear, stated objective — to retaliate against al-Qaeda and prevent a repeat of the Sept. 11, 2001, attacks.

Yet the interviews show that as the war dragged on, the goals and mission kept changing and a lack of faith in the U.S. strategy took root inside the Pentagon, the White House and the State Department.

Fundamental disagreements went unresolved. Some U.S. officials wanted to use the war to turn Afghanistan into a democracy. Others wanted to transform Afghan culture and elevate women’s rights. Still others wanted to reshape the regional balance of power among Pakistan, India, Iran and Russia.

“With the AfPak strategy there was a present under the Christmas tree for everyone,” an unidentified U.S. official told government interviewers in 2015. “By the time you were finished you had so many priorities and aspirations it was like no strategy at all.”

The Lessons Learned interviews also reveal how U.S. military commanders struggled to articulate who they were fighting, let alone why.

Was al-Qaeda the enemy, or the Taliban? Was Pakistan a friend or an adversary? What about the Islamic State and the bewildering array of foreign jihadists, let alone the warlords on the CIA’s payroll? According to the documents, the U.S. government never settled on an answer.

As a result, in the field, U.S. troops often couldn’t tell friend from foe.

“They thought I was going to come to them with a map to show them where the good guys and bad guys live,” an unnamed former adviser to an Army Special Forces team told government interviewers in 2017. “It took several conversations for them to understand that I did not have that information in my hands. At first, they just kept asking: ‘But who are the bad guys, where are they?’ ”

The view wasn’t any clearer from the Pentagon.

“I have no visibility into who the bad guys are,” Rumsfeld complained in a Sept. 8, 2003, snowflake. “We are woefully deficient in human intelligence.”