The Rules of War do not require you to wear a uniform 24 hours a day, seven days a week if you are not, at the time, actually engaged in combat. If so, any U.S. off-duty soldier captured while on liberty or in a rear recreation area in civilian clothes or buck naked in a shower could be denied POW status with a legal excuse to back it up.
That's not a good precedent to establish.
The Hague Convention codified the qualifications for belligerent rights as follows:
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REGULATIONS RESPECTING THE LAWS AND CUSTOMS OF WAR ON LAND
SECTION I
ON BELLIGERENTS
CHAPTER I
The Qualifications of Belligerents
Article 1.
The laws, rights, and duties of war apply not only to armies, but also to militia and volunteer corps fulfilling the following conditions:
To be commanded by a person responsible for his subordinates;
To have a fixed distinctive emblem recognizable at a distance;
To carry arms openly; and
To conduct their operations in accordance with the laws and customs of war.
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In the first point, Saddam is in the clear. He is the leader.
However, if Saddam is connected to any car bombing where the driver posed as a civilian or in any bombing that attacked civilian targets, such as the Red Cross compound, then Saddam would have violated some or all of the last three conditions for qualification as a legal combatant with POW rights.
There must be special rules for a captured Head of State and Supreme General.
Wearing a uniform.
-or- Bearing arms openly in any engagement, even without a uniform (specifically includes guerillas)
In addition, wearing civilian clothes and *not* being armed, grants civilian non-combatant status instead, *if* one has refrained from overt, hostile acts, and obeys any laws promulgated by an occupier.
You lose PW or non-combatant status (one or the other) only if you combine -
1. not being in uniform with either
2. carrying concealed arms or
3. committing overt, hostile acts without being armed
The last would include, for example, conveying military information to belligerents after scouting, unarmed, in civilian clothes.
From what we are told, Saddam was armed and made no attempt to conceal the fact when he gave himself up, and voluntarily surrendered his weapon. That would normally be enough to make him a PW. Obviously, he is wanted for crimes against humanity, which PW status does not protect one against. But he has to be tried for those.