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The Americans argued that captured members of al-Qaeda do not fall into any of these categories, saying that al-Qaeda members don't wear uniforms ("fixed distinctive sign") or obey the laws of war. Rumsfeld labeled them "unlawful combatants," and said the rules of the Geneva Convention did not apply.
An American military pamphlet on the law of war provides this definition: An unlawful combatant is an individual who is not authorized to take a direct part in hostilities but does. ... Unlawful combatants are a proper object of attack while engaging as combatants. ... If captured, they may be tried and punished. As examples, the pamphlet mentions civilians who engage in war without authorization; non-combat members of the military, such as medics or chaplains, who engage in combat; and soldiers who fight out of uniform. In the Second World War, the United States captured eight German saboteurs who were out of uniform and executed six of them.
However, under the Geneva Conventions, it's up to an independent judge to determine the status of the "detainees," not whoever detains them. As well, Canadian regulations on prisoner-of-war status dictate that detainees must be brought before a military tribunal to determine whether they're prisoners of war or not.
http://www.cbc.ca/news/background/iraq/genevaconventions.html