One of my most incongruous memories of covering Donald Rumsfeld, who died this week at the age of 88, was watching the US defence secretary receive a lesson in Irish revolutionary history at the Shannon airport bar.

Returning from a trip to China, we had stopped at Shannon to refuel. Telling him that it was rude not to drink with an Irishman in his own country, I convinced Rumsfeld to join the reporters and aides for a drink.

As Rumsfeld drank Irish coffee, the barman explained how one of the reporters had committed a sin by ordering a “Black & Tan” — the name given to a controversial British police unit during the Irish war of independence, but a drink in the US. It was a light-hearted moment for someone whose own gaffes frequently landed him in trouble.

During his career, Rumsfeld served in myriad roles, including as a congressman, Nato ambassador and White House chief of staff to Gerald Ford, who later made him defence secretary. When George W Bush appointed him to the same role in 2000, Rumsfeld made history by having been both the youngest and oldest Pentagon chief — and the only person to hold the role twice.

Bush appointed Rumsfeld both because of his security credentials but also because as chief executive of GD Searle, a pharmaceutical company, he had earned plaudits for effectively restructuring a big organisation, and Bush wanted someone to reform the Pentagon. But he was cast into a very different role after the 9/11 terror attacks, which led to the US invasions of Afghanistan and Iraq.

By the time I started covering Rumsfeld in 2004, the Iraq war was almost a year old. The jubilation that had come with the capture of Saddam Hussein was morphing into frustration as a potent insurgency arose across the country.

After being feted for the speed at which the US had toppled Saddam, Rumsfeld became the poster child for everything that was going wrong in Iraq, much of which was due to a lack of planning and insufficient US troops.

Yet even before Iraq descended into a treacherous quagmire, he would deflect hard questions with his verbal dexterity. In a city of egos, he stood out as someone who was so confident that he would never accept blame.

When widespread looting swept Iraq, his response was: “Freedom is untidy” and “stuff happens”. Insurgents were “dead-enders”. When a soldier asked why the US had been so slow to armour its Humvees, he replied: “You go to war with the army you have.” In approving controversial interrogation techniques for Guantánamo Bay, Rumsfeld, who used a standing desk, scribbled: “I stand for 8-10 hours a day. Why is standing limited to 4 hours?”

A former Navy pilot and wrestling champion at Princeton, Rumsfeld was hyper competitive. An avid squash player, he made his aides play hard ball — a form of the game that requires less running to give him an advantage — and would playfully post the score on his office door when he won.

While it was hard to separate the man from the defence secretary, Rumsfeld had charm and wit. But he also had a dark streak that saw him bully people, including his generals. His skill as an infighter once led Richard Nixon to describe him as a “ruthless little bastard”.

I experienced his abrasiveness on a trip to north Africa when he castigated me over a blog that, he claimed, made it sound like he travelled in luxury. When a reporter later joked that Rumsfeld was scared to play me at squash, he exclaimed disdainfully: “What, this jackass who wants to be Financial Times food editor?” (I had interviewed him for Lunch with the FT just before the trip.) The real reason for his anger, I later learned, was that I had written that the military doctor distributed sleeping pills to the press corps on the plane. He never let me travel with him again.

But while Rumsfeld, as the former CEO of a drug company, was very worried about the optics of giving out the sleep aids, he showed little concern about his image when it came to the treatment of Guantánamo detainees, or the fact that Iraq had none of the weapons of mass destruction that the US said justified the fateful invasion.

One of his most vocal critics was Senator John McCain, who said he would “go down in history as one of the worst secretaries of defence”. He is often compared with Robert McNamara, the Vietnam war-era defence secretary, who also entered the public pantheon of villainous defence officials. Yet, unlike McNamara, who later showed some remorse, Rumsfeld never showed regret for the Iraq war, which claimed the lives of almost 500,000 people and left the country in chaos from which it has yet to recover.

Whether Rumsfeld ever expressed remorse in private will remain, in his own words, “a known unknown”.