Since they are not a national military, and wear no uniforms, I would say no.
Does this mean that they can be legitimately destroyed, using poison gas or biological agents? Also, since they wear no uniforms, can they be executed as spies?
If we decide to engage these terrorists, do any rules of war apply?
Common Article 3 of the Geneva Conventions provides basic human rights protections to anyone who is captured on the battlefield. That includes protections against murder and inhumane treatment, and for access to medical treatment.
Not having a uniform on does not free countries up to waive the laws of armed conflict. For example, attacks on an enemy must still be proportional and have military relevance. Further, virtually all nations have agreed to ban the use of chemical and biological weapons as a blanket policy, including the possession of these types of weapons. Therefore, there is no legal basis to deploy chemical or biological weapons at all, in any case.
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I doubt that ISIS would get significant protections from the Geneva conventions. They aren’t a recognized state, haven’t ratified the conventions, and don’t accept and apply the conventions to their armed forces. Common Article 2 states that Geneva Conventions only apply to conflict between states that have ratified the Geneva Conventions, or to organizations that haven’t formally ratified the conventions but do accept and apply them. Ravenman mentions Common Article 3, but that only applies to conflicts within one nation, once you have multiple nations article 3 is irrelevant. ISIS combatants would not qualify for POW status, but the capturing power would be required to try them under either that power’s laws or the laws of the state the conflict was in. In practice a lot of them would likely end up in a Guantanamo Bay situation.
Use of poison gas and biological agents is forbidden under various other treaties, and would be a monumentally stupid decision for a whole host of reasons. No major power is going to open that mess.
Just because they aren’t ‘following the rules’ of the Geneva Convention in regards to warfare doesn’t mean an adhering country can break the rules in terms of fighting them to the point of using chemical or biological weapons. That would be a war crime in and of itself regardless of who you use these weapons against. As stated above it means they are sometimes not entitled to official POW status as they are not considered legitimate ‘soldiers’. Wikipedia further states that:
According to Article 4 of the Third Geneva Convention of 1949, irregular forces are entitled to prisoner of war status provided that they are commanded by a person responsible for his subordinates, have a fixed distinctive sign recognizable at a distance, carry arms openly, and conduct their operations in accordance with the laws and customs of war. If they do not meet all of these, they may be considered francs-tireurs (in the original sense of “illegal combatant”) and punished as criminals in a military jurisdiction, which may include summary execution.*
*****emphasis mine
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