By Nick Childs
World affairs correspondent, BBC News
Simply by his passing, the exit of Donald Rumsfeld will do much to change the tone of the debate over Iraq.
Robert Gates (right) will face the same decisions as Donald Rumsfeld
He had so come to symbolise the failings of US policy in Iraq, had appeared so trenchant and domineering, and had become such a hate figure for the Bush administration's critics, that the arrival of Robert Gates, a very different character without Mr Rumsfeld's political baggage, inevitably offers the prospect of greater flexibility.
As well as the arrival of Mr Gates, there are other key changes in US personnel on Iraq in the offing.
The Pentagon's two top generals dealing with the country - General John Abizaid, the head of US Central Command, and General George Casey, in charge in Iraq itself - are both expected to move on in the coming months.
So, too, it is thought, may the current US ambassador in Baghdad, Zalmay Khalilzad.
Iraq Study Group
And there is a consensus now that some kind of change is necessary - just not on what that change should be.
The anticipated report of the Iraq Study Group, headed by the former US Secretary of State, James Baker, could provide the bipartisan political cover for change. (Mr Gates is a member of the group, suggesting Mr Bush is inclined to listen to its recommendations when they come.)
The Pentagon's top brass may also feel freer to offer Mr Gates new options for the US military presence in Iraq than they did Mr Rumsfeld.
The White House may be more open to troop reductions in Iraq
That all may make the Bush administration more open to the idea of accelerating the phased reduction in the current US troop strength in Iraq.
And that could have implications for the British military presence in the south.
The British government faces most of the same pressures - political as well as military - that President Bush and his advisers face.
Their shared problem is that the viable options are all limited and risky - and both US and British influence in Iraq itself continues to dwindle.
So the prospect of drastic change - particularly in the number of US and British troops deployed in the short term - is not great.
And, while Mr Rumsfeld has had huge criticism heaped on his shoulders, there is also plenty of responsibility to be shared around.
That does not change with his departure.
Specifically, the US Vice-President Dick Cheney, the ultimate administration hawk, is still there.
And, at the Pentagon, the uniformed military itself must share some of the blame for failing to get to grips with post-invasion Iraq.
In more general terms, where does the Pentagon go from here?
Long before Iraq, Donald Rumsfeld had crossed swords with the US top brass.
He was given the job of reasserting civilian control of the military, and of revamping an institution seen as resistant.
The Pentagon has been seen as resistant to change
He was determined to run the Pentagon, rather than have it run him.
That and his abrasive style meant he inevitably clashed with the generals.
Ironically, he looked on the way out in first few months of the Bush administration, as his efforts at reform faced increasing resistance.
It was the attacks of 11 September 2001 which may have saved him politically.
Robert Gates has a different style and is more consensual.
His arrival may mean the Pentagon will have less of a voice in the administration's policy counsels, and that the balance of debate in the White House has shifted away from the hawks.
The Rumsfeld ideas for transforming the military into a more high-tech force, relying less on troop numbers for effect, may look discredited in the light of Iraq.
He will be forever associated with the decision to send only a small invasion force to Iraq - fatally too few troops in the eyes of most critics.
But his reforms of the US military presence around the world, reducing troop numbers in Europe and Asia, were widely seen as overdue.
And, in general, his push to make the US military - and the US Army in particular - lighter, more agile, and more deployable to face new kinds of threats will go on.
With the massive expenses of Iraq, little likelihood of even more defence spending, and all the other pressures on the Pentagon budget for new weapons, Mr Gates will face many of the same difficult decisions that Donald Rumsfeld would have faced over the next two years.