US Defence Secretary Donald Rumsfeld has been characteristically outspoken in his support for a tough approach to Iraq.
Rumsfeld has attempted to transform the US military
Along with fellow hawks, the man at the helm of the Pentagon throughout Washington's response to the 11 September attacks has appeared sceptical about the role of weapons inspectors and suspicious of Iraqi requests for dialogue.
He has suggested that Iraq has knowingly allowed militants to develop chemical and biological weapons and accused it of lying about its own weapons programme.
If you are not criticised, you are not doing your job
Donald Rumsfeld, from Rumsfeld's Rules
Amid discussion about whether the US would attack Iraq without UN backing or international support, Mr Rumsfeld compared the US president's stance against Saddam Hussein to Winston Churchill's "lone voice" calling for action against Hitler.
He has also dismissed French and German scepticism over the US pressure on Iraq as the voice of "old Europe".
Mr Rumsfeld has said international unanimity is less important than "making the right decisions".
He also said he thought other countries would fall in behind once the US committed itself to a military course of action.
1962: House of Representatives
1969: Various roles serving Nixon administration
1973: Ambassador to Nato
1974: Chief of staff to Gerald Ford
1975: Defence secretary to Gerald Ford
1983: Visits Iraq and meets Saddam Hussein as US repairs relations with Baghdad
2001: Defence secretary
Mr Rumsfeld has faced the task of redirecting the strategy and arsenal of the world's largest military power to take on the invisible, stateless enemy of global terrorism.
But analysts say he has become frustrated with the pace of the war on al-Qaeda.
He has also strongly defended George Bush's statements against the "axis of evil" made up of Iran, North Korea and Iraq.
A long-time advocate of plans for a national missile defence system, he has warned against such "rogue states" for some time, particularly in a 1998 report by an independent commission which he chaired.
Reform of the military
Mr Rumsfeld has tried to transform the US military.
He has said that 11 September taught the US that its adversaries are changing.
"They're watching us; they're studying how we were successfully attacked, how we responded. And they're looking for ways that we may be vulnerable in the future. And we stand still at our peril."
He's the most ruthless man I ever met... and I mean that as a compliment
Henry Kissinger on Donald Rumsfeld
In line with this, Mr Rumsfeld cancelled a $9bn contract for the Crusader army artillery weapon, despite protests from the army and from weapons manufacturers.
Mr Rumsfeld argued that it was a weapon designed for old-fashioned conventional warfare, such as repelling a Soviet invasion of Germany.
Ambition and ability
Correspondents say the 70-year-old former Princeton University wrestler is a tough and determined character.
Mr Rumsfeld wrote a pamphlet known as Rumsfeld's Rules - famous in US political circles - which collects nearly 30 years' worth of quotations and reflections by himself and others.
In a chapter titled "Keeping Your Bearings in the White House", one entry reads: "If you are not criticised, you are not doing your job".
Another, in a chapter headed "Serving in Government", states: "If in doubt, don't. If still in doubt, do what's right".
Mr Rumsfeld's deputy, Paul Wolfowitz, once described his boss as "a constant, active source of energy... he generates a mini-storm wherever he goes".
Henry Kissinger once said that Mr Rumsfeld was the most ruthless man he knew.
He added that he was a "skilled full-time politician-bureaucrat in whom ambition, ability and substance fuse seamlessly".
Like Dick Cheney, Mr Rumsfeld is in a top job for the second time round - he served as the country's youngest ever defence secretary from 1975-77 under President Gerald Ford.
He spent three years in the US navy before starting out his political career as an assistant to a congressman.
Twenty years and several jobs later, he was appointed defence secretary for the first time - nearly 25 years before he took the post under George W Bush.
Much of the time between was spent in big business, including stints as chief executive officer of pharmaceutical company GD Searle & Co, CEO of General Instrument Corporation and Chairman of Gilead Sciences.
But he also continued to advise Republican administrations and exercise his influence over issues of defence throughout the 1980s and 1990s.