Donald Rumsfeld’s name will forever be associated with the biggest military fiasco in US history, the 2003 invasion of Iraq in pursuit of non-existent weapons of mass destruction, alongside the widespread use of torture that has dogged America’s reputation ever since.

It is not just the poor decisions he made as defence secretary for which Rumsfeld will be remembered, but also his efforts to cover up inconvenient facts that did not align with his version of reality.

Documents surfaced after the invasion that showed that Rumsfeld was quite aware of the gaping holes in the intelligence about Iraqi WMD, but he consistently presented the claims to the public as if they were cast-iron certainties.

He also played down the growing insurgency against the US-led occupation after Saddam Hussein’s fall, dismissing the collapse of law and order in Baghdad with the insouciant phrase “stuff happens”, which would go on to haunt him for the rest of his life.

His reluctance to heed warnings that did not fit in with his worldview alienated the generals and the military rank and file. His insistence there was no serious threat in Iraq contributed to the fact that the US military was driving around in lightly armoured Humvees a year after the invasion.

In November 2006, the Army Times took the unusual step of calling for his resignation.

“Rumsfeld has lost credibility with the uniformed leadership, with the troops, with Congress and with the public at large,” an editorial said. “His strategy has failed, and his ability to lead is compromised. And although the blame for our failures in Iraq rests with the secretary, it will be the troops who bear its brunt.”

When George W Bush appointed Rumsfeld secretary of defence in 2001, it was widely thought he and his fellow veteran from the Gerald Ford administration, Dick Cheney, would be a moderating influence on a callow and ideological president.

After the 9/11 attacks, however, Rumsfeld and Cheney, together with Rumsfeld’s deputy, Paul Wolfowitz, emerged as radical warmongers driven by fear of worst-case scenarios with little or no basis in reality - in particular the idea that Saddam was allied with al-Qaida, had chemical and biological weapons, and was on the brink of building nuclear warheads.

Rumsfeld became famous for his philosophical musings about the distinction between “known knowns, known unknowns and unknown unknowns”. What he did not mention was almost all the intelligence of Iraqi WMD fitted into the second and third category.

In September 2002, the intelligence director for the joint chiefs of staff reported that: “We’ve struggled to estimate the unknowns ... We range from 0% to about 75% knowledge on various aspects of their program.”

“This is big,” Rumsfeld said in an appended comment on the report, but it made no impact on the certainty of his continued claims. In January 2003 he declared the Saddam “has large, unaccounted for stockpiles of chemical and biological weapons, including VX, sarin, mustard gas, anthrax, botulism and possibly smallpox”.

“And he has an active program to acquire and develop nuclear weapons,” Rumsfeld said.

Frustrated by the failure of the US intelligence community to come up with reporting that confirmed his beliefs, Rumsfeld launched a parallel intelligence collection mechanism in the Pentagon that was heavily influenced by Iraqi exiles led by Ahmed Chalabi.

Those same exiles also helped convince Rumsfeld, Wolfowitz and Cheney that US forces would be hailed as liberators after the fall of Saddam, laying the groundwork for the establishment of Iraqi democracy. In retrospect Rumsfeld was blithely optimistic about the conflict. In November 2002, he admitted he did not know if it would take five days, five weeks or five months, while adding “it certainly isn’t going to last any longer than that.”

Rumsfeld began the US involvement in “enhanced interrogation techniques”, now widely recognized as including torture, with the same airy confidence. In a characteristically terse scrawl on one memo about techniques in late 2002, he asked why enforced standing should be limited to four hours while he stood at his desk for eight to 10 hours.

Another Rumsfeld legacy that continues to burden the US government two decades on is Guantánamo Bay, which he argued at the time was “the least worst place” to hold terrorist suspects and battlefield captives beyond the reach of US legal protections. Successive administrations have sought to close the prison camp which has become an embarrassment and an obstacle to getting justice for the victims of 9/11. The use of torture during detention has tainted evidence, preventing the trial of the key defendants from even starting.

In his memoir, Rumsfeld grudgingly accepted that he made “a few misstatements” in one of his assertions about Iraqi WMD sites and said he was “surprised and troubled” to learn after the fact about the lengths US interrogators had gone to. He described the abuses at Abu Ghraib prison in Baghdad as the darkest hour of his Pentagon career.

He presented such excesses as glitches rather than inevitable outcomes of his policies. History is unlikely to be as forgiving.

Julian Borger in Washington