The best way to picture the power structure in Saddam Hussein's Iraq is to imagine a series of widening circles, with the figure of the president at their centre.

Those closest to the heart of power are those whom Saddam Hussein trusts the most - though never unconditionally - because he knows that they are so closely identified with him that they have nowhere else to go.
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Beyond this circle are those who receive special favours and in return are expected to devote their lives exclusively to the service of the president.

These men are drawn heavily from the allied clans and tribes of the Sunni Arab north-west of Iraq, but also include those who have made their careers within the hierarchy of the Baath party, as well as state servants.
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Further out, beyond these figures and more distant from the president, but nevertheless integrated into the webs of patronage, are those who are considered useful for social control.

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This pattern is repeated throughout the system: all must compete for the president's favour and no individual is ever allowed to believe that he has a right to the favours bestowed.

Even Saddam Hussein's closest family - his half-brothers, his sons, his cousins - need to be reminded of this from time to time, sometimes violently.
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More threatening to the system as a whole, however, and more relevant to the present situation, is the emergence of a possible new order or alternative system of power at the same time as Saddam Hussein's ability to maintain oversight and control is perceived to be weakening.

It is then that members of the outer circles will be tempted to defect to alternative centres of power, if they feel that they can do so without risk of reprisal.

To some degree this has happened during the past 12 years of the existence of the Kurdish regional government.

In the present crisis, the US administration must hope that its threats of imminent invasion and the overthrow of the regime will lead to cracks of a similar kind within the circles closer to the heart of power.

» BBC NEWS | Middle East | Analysis: How Saddam rules Iraq