But a strange transformation has come over him, and Americans' view of him, since the cataclysmic events of September 11.
The hardest of the Bush Administration's hard men has visibly and audibly softened. He has exhibited a human streak that was rarely evident before.
His crustiness seems no longer that of the impatient didact, but a carapace, deliberately developed, to protect a vulnerable heart.
Rumsfeld is not the only member of the Bush Administration whose public and political stock has soared since the terrorist attacks, with the biggest beneficiary being the President, but he is, perhaps, the most surprising.
When the hijacked Boeing 757 ploughed into a wing of the Pentagon, Rumsfeld was in his office not so very far away, watching live television coverage of the attacks on the New York trade center towers. Neither he, nor any of his staff, had received any warning that the next hijacked plane might be headed for them.
Rather than flee to the basement, the doughty Rumsfeld made a beeline for the site of the crash, established that the centre of the superpower's defences had been hit by a passenger plane (however improbable that scenario might have seemed barely one hour before) and busied himself with the rescue of those who had been injured.
The Defence Secretary then returned to his office and held the fort, refusing to evacuate to more secure accommodation. His place, he said afterwards, was at his desk in the Pentagon.
The first to call a news conference that day - even as the President was being shuttled around the country by the Secret Service - Rumsfeld gave straight answers to awkward questions, while adhering to what by then appeared to be a policy not to divulge the likely scale of the casualties.
He also passed the test of never appearing to be grabbing for power. He deferred to the President, he never said: "I'm in charge." He showed himself to have a cool head in a crisis as well as a keen appreciation of what was appropriate.
"Rummy" subsequently distinguished himself with an elegant morale-boosting call to his staff, while also sending private messages to the bereaved and keeping the Pentagon running.
He gave most of the week's Pentagon press briefings himself, in the same matter-of-fact but humane style in which he had started. If a compliment was offered, he insisted modestly that he was just doing his job.
To the people who know him best - among them Vice-President Dick Cheney, a former Defence Secretary himself - this other side of Rumsfeld was never a secret. They were more surprised by how long it took to emerge.
They knew the Defence Secretary as a man who did not suffer fools gladly, but who was very far from the humourless bureaucrat he so often appeared.
In his first years at the Pentagon (1975-77), he formulated what started to circulate as "Rumsfeld's Rules" - a collection of wry personal observations and advice for public servants, especially those in authority.
Among these rules - which Rumsfeld's office agreed to circulate, by popular request, soon after he arrived at the Pentagon in February - are such trenchant, and freshly pertinent, points as this: "Don't divide the world into 'them' and 'us'. Avoid infatuation with or resentment of the press, the Congress, rivals or opponents. Accept them as facts." And this: "If you are not criticised, you may not be doing much." Or: "Look for what's missing. Many advisers can tell a President how to improve what's proposed, or what's gone amiss. Few are able to see what isn't there." Such as, he might now be moved to comment, US passenger planes used by terrorist hijackers as weapons of war.
The portrait of "Rummy" that emerges from his rules is of a man simultaneously modest and aware of his worth, a man with a dry sense of humour that he is accustomed to reserving for family and friends, but someone who also takes his responsibilities, whether in the private or public sector, extremely seriously.
One piece of advice he also dispenses is: "Be able to resign. It will improve your value to the President and do wonders for your performance."
Thanks to his high-flying early career in government and his subsequent success in the private sector, Rumsfeld can afford to take his own advice, should the need arise.
A native of Chicago and a graduate of Princeton University, which he followed with three years as a naval aviator, he alternated politics and business in his early career, succeeding in both.
He moved, seemingly without effort, from investment banking into the US Congress as a Republican from Illinois, at the young age of 30.
He jumped directly into the Nixon Administration as an assistant and then counsellor to the President, before being appointed ambassador to Nato for two years.
That experience stands him in good stead in the extraordinary circumstances that the US finds itself. At the time, it meant that he escaped the shadow of Richard Nixon before disgrace descended on that presidency.
Rumsfeld was recalled to Washington by Gerald Ford to oversee his transition to government. The post of White House chief of staff predictably followed and then - far less predictably - at the age of 44, the job of Defence Secretary.
Under Jimmy Carter, Rumsfeld moved into the private sector, as a highly prized (and paid) executive, thanks to his combination of White House and Pentagon experience.
He is credited with turning around a series of companies, starting with the pharmaceutical firm GD Searle and moving on to high technology, communications and scientific ventures.
At the same time, however, he kept a foot in the public camp, serving on numerous commissions and maintaining his defence expertise with a special focus on the possibilities afforded by offence and defence in space.
It was this expertise, and his reputation for straight-talking and seriousness, that recommended him to someone who, on the face of it, was hardly a political ally: Bill Clinton. In the last year of his presidency, Clinton asked Rumsfeld to chair a key inquiry to assess the feasibility of and need for a national missile defence programme - the "son" of President Reagan's "star wars".
The Rumsfeld report, as the result of the inquiry was immediately known, supported the building of a space defence system on the two counts of need and feasibility, and increased congressional pressure on Clinton to press ahead with the venture.
A series of failed tests, however, gave Clinton - never completely sold on the idea himself - the pretext he needed to hold a final decision over to the next administration.
Rumsfeld's now passionate support for missile defence was a central consideration for Bush when he came to trawl for a Secretary of Defence.
Committed support for that one policy, along with a blameless private life and a like-minded approach to Republican politics, were what Bush was looking for in his first defence secretary. But it was only when at least two other candidates failed those tests that he turned to the then retired Rumsfeld.
At his swearing-in, Rumsfeld said he had little hesitation in accepting the post. He was too discreet to mention, though many others did, that he is one of several Bush cabinet members who will lose large amounts of potential income from investments and consulting by accepting a government salary.
But, as is clear from Rumsfeld's Rules, the Pentagon chief values independence of mind and the freedom to resign - and these are principles that his money would allow him to observe, should any conflict arise with the Administration.
Until the unheralded terrorist attacks on New York and Washington, there was a gathering sense that conflict with Congress might be in the offing.
Rumsfeld had firm ideas about what he wanted to achieve, but the political skills he might have learned as a congressman those many years ago seemed to have left him. At successive congressional hearings on strategy and budgets, he appeared opinionated and brusque.
When Bush failed to allocate the Pentagon as much money as Rumsfeld had earlier appeared to promise, support for him diminished even among his own Republicans.
The new Defence Secretary was direct in saying that he thought the US armed forces could do much more with their vast allocation; the underlying problem, he insisted, was not so much the budget - though it was too small - as the outdated uses to which it was put.
The threat of changed priorities, including an end to the much-cherished principle that the US should be equipped to fight two large-scale wars on two fronts at once, drew fierce opposition from service chiefs.
It provoked the particular ire of the Army and Navy top brass, who envisaged their manpower and money being siphoned off to the Air Force and a "star wars" missile defence system that might well not work.
The notion of Rumsfeld as a zealous reformer also came as a shock to those who had known him at the Pentagon before. They shared Kissinger's view of him as "a skilled full-time politician-bureaucrat" who "thwarted new diplomatic initiatives or military moves by a rigorous insistence on bureaucratic procedures and playing devil's advocate to every new proposal".
What had not changed with the years, however, was Rumsfeld's hawkish view of the world, which seemed to see enemies, rather than allies, beneath every stone.
The events of September 11 have changed the United States' perception of the world and its own security requirements just as surely as the collapse of communism did a decade ago.
The context of every defence debate, whether about spending priorities or "star wars" or enemies, has changed, too.
The country now finds itself, in the words of its President, a nation at war.
Nowhere will the change be felt more keenly than in the Pentagon and, as Rumsfeld showed in the first hours after the terrorist strikes, no one handles himself better in the principled pursuit of conflict than he does.
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INDEPENDENT
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