HIS press conferences used to be compulsive viewing — full of wisecracks and confidence, the most combative and controversial US Defence Secretary since the Vietnam War. Journalists indulged him. The American public loved him.

Suddenly, in the space of 72 hours, Donald Rumsfeld appears to be a changed man. No other American politician has invested so much personal capital in the war on Iraq and yesterday, after the resignation of a key adviser and growing sniping from the military establishment, the strain was beginning to show.

The lightness of touch was gone. It was not just his unexpected and provocative warnings to Syria and Iran against aiding Iraq that suggested a man under pressure. He was tetchy, impatient, at times angry and clearly frustrated.

Asked about his growing band of high-ranking critics, Mr Rumsfeld, never one to flinch from a fight, surprised even some staunch supporters in his answer. A month ago, perhaps, he would have laughed it off. Yesterday he replied: “The war plan is (the coalition commander) Tommy Franks’s war plan.”

If Mr Rumsfeld had had his way a year ago, his Pentagon critics maintain, there would be only 75,000 troops trying to overthrow President Saddam Hussein, and not the 300,000 demanded by General Franks. That answer, they say, smacks of a man passing the buck. Mr Rumsfeld, a relentlessly pugnacious operator, a former fighter pilot and All Navy wrestling champion, is still talking tough. But as he appeared before the press corps yesterday, the Teflon-like irrepressible confidence that had clung to him from the moment that he rushed from his Pentagon office on September 11 to tend to the wounded — a moment that transformed him overnight into a national hero — had dissipated. At one point, asked if the Pentagon was deliberately withholding US casualty figures, he banged the podium angrily as he issued a denial.

The sense of a politician suddenly on the back foot was heightened by the resignation on Thursday of Richard Perle, a key Pentagon adviser and a close friend of Mr Rumsfeld. Mr Perle, a highly influential Washington hawk and the intellectual driving force in formulating the Bush Administration’s policy on overthrowing Saddam, stepped down as chairman of the Pentagon’s Defence Policy Board amid allegations of a conflict of interest. It will not diminish Mr Rumsfeld’s determination to destroy Saddam, but the timing of Mr Perle’s departure, in a week when the military and ideological rationale for invading Iraq came under sustained criticism, was inauspicious.

The greatest danger for Mr Rumsfeld comes not from a sceptical press, however, but from the military itself. He is under withering and sustained fire from some of America’s most respected and senior former wartime generals. No longer briefing journalists anonymously, a favoured tactic among Mr Rumsfeld’s Pentagon adversaries, the recently retired top brass have broken cover to accuse him of ignoring warnings that tens of thousands more troops were needed to attack Iraq.

“Shock, awe, and over-confidence,” was how Ralph Peters, a retired officer, writing in The Washington Post, described Mr Rumsfeld’s policy. “Military planners have argued for months that more and heavier ground forces were needed . . . Rumsfeld personally and repeatedly rejected calls for the deployment of additional army divisions . . . Rumsfeld was out to prove a point. In his vision of the future, one shaped by technocrats and the defence industry, ground forces can be cut drastically. But those who want to wage antiseptic wars for political purposes should not start wars in the first place.”

Former Gulf War commanders have joined the fray. On Monday the retired US Army General Barry McCaffrey said that Mr Rumsfeld was told to send more troops to Iraq. “I think he thought these were generals with feet planted in World War Two, that didn’t understand the new way of warfare.”

President Bush is the Commander-in-Chief, but on the ground, in its execution, this is Mr Rumsfeld’s war. Its outcome will determine not only his political future, but the fate of his bruising crusade to confront the Pentagon’s military wing and transform the way in which America fights its enemies in the 21st century.

He was elected to Congress in 1962, Ambassador to Nato under President Nixon, Chief of Staff and then Defence Secretary under President Ford at the age of 43. Now, at 70, Mr Rumsfeld is both the youngest and oldest Defence Secretary in American history. He is also the only member of the Administration who has met Saddam, shaking his hand in Baghdad in 1983 as President Reagan’s special envoy to the Middle East when Iraq was viewed in Washington as a secular ally against Iranian fundamentalism.

A product of the Cold War, Mr Rumsfeld is of a generation whose outlook and attitudes appeared to be outdated before September 11. He came to the job determined to transform the military into a lighter, faster-moving fighting force, more reliant on technology, and to confront a Pentagon culture that had become risk-averse since Vietnam.

Immediately he gained a reputation for being abrasive, sometimes abusive to underlings, and frequently dismissive of senior military officers’ advice. By the summer of 2001 he was widely expected to resign.

Then came September 11, and his fortunes were transformed. In Afghanistan, a war won with airstrikes guided by small teams of special forces and ground battles led by militia, Mr Rumsfeld’s vision appeared vindicated.

So, too, did his unbridled enthusiasm for a pre-emptive, aggressive American foreign policy. After September 11 he embodied a confidence born of unchallenged US might.

On his Pentagon desk is a bronze plaque quoting Teddy Roosevelt: “Aggressive fighting for the right is the noblest sport the world affords.”

It is worth noting the quote displayed in the office of Colin Powell, the Secretary of State, whose doctrine of overwhelming force has been shunned by Mr Rumsfeld: “Of all the manifestations of power, restraint impresses men the most.”

Times Recommends

• Rio Tinto four held as China prepares case
• Diplomats must keep heads down and trousers up
• When in Rome should one make like Berlusconi?