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It is not infrequent that rules of war are not found in the Geneva Conventions, as expected, but rather in the International Committee of the Red Cross's online database:

For example, there are Geneva conventions about not blowing up civilian targets like hospitals, but if you look for something that tells you "don't shoot at civilians", you will instead arrive at the ICRC's Rule 1:

Rule 1. The Principle of Distinction between Civilians and Combatants

Rule 1. The parties to the conflict must at all times distinguish between civilians and combatants. Attacks may only be directed against combatants. Attacks must not be directed against civilians.

What gave rise to this Customary International Humanitarian Law? Did countries sign up for it in some way (as for example when joining the UN - it doesn't seem to be in the UN Charter)? Is there a relationship to the Geneva Conventions (it seems pretty complementary to it)?

p.s. I am not criticizing this body of Customary IHL or doubting its universal applicability, just curious on its origin and status. Yes, I kinda see what customary might mean in a common law national system. This is international law however.

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Unlike domestic law, there’s no international authority that gets to decide what international law says. International law, such as it is, is largely based on “this is simply how countries act.” While treaties are an important part of international law, restricting yourself to treaties would lead you to the conclusion that international law largely did not exist until after World War II.

Instead of being based on treaties, customary international law is created when countries pretty consistently consider something a legal obligation. This isn’t nearly as simple as “Treaty X says parties must do Y.” Analyzing the customary law applicable to a situation means looking to any recent situations where anything similar happened, national laws, public statements, etc. The result isn’t the one true answer, either —- different people can disagree about whether something has achieved the level of consistent practice needed to become customary law, or whether countries were truly following it because they thought of it as law. (For instance, no country uses glitter bombs in war, but that’s because it’s stupid and not because of any legal issue).

A lot of widely-cited treaties are actually just codifying preexisting customary international law. For instance, the Vienna Convention on Diplomatic Relations didn’t create diplomatic immunities. All countries already recognized the idea that international law protected diplomats from the jurisdiction of the host country. Long before the UN Convention on the Law of the Sea, it was universally acknowledged that ships on the high seas were generally not subject to the jurisdiction of anyone but the flag state.


A lot of rules of international humanitarian law started as customary law. A lot of them have now been codified, but those treaties aren’t universally accepted. The Geneva Conventions of 1949 are essentially universal, but they protect a fairly narrow range of people: basically the sick and wounded, medics, prisoners of war, and residents of occupied territories. Older treaties might only be between a small number of major Western powers of the time. Additional Protocols I and II cover many basic rules, but some major powers have not ratified those. And even with those treaties, interpreting them often requires looking at the customary law they were created to codify.


Customary IHL is one of the most important components, but figuring out what it says takes a lot of hard scholarship. The ICRC decided to do a study in 2005 to compile generally accepted rules and relevant state practice to serve as a reference. That’s what their customary IHL database consists of. No country has agreed to it as a treaty, but it is a widely-respected compilation of what in fact is customary law.

That’s not to say everyone agrees. For instance, the US accepts many of the rules in the ICRC’s study as customary IHL, but it had issues with some of the study’s methodology. The ICRC’s study is at best a secondary source of international law (writings of eminent legal scholars), so the fact that a state disagrees doesn’t mean the state is wrong. The rule against targeting civilians is universally accepted as international law, but the rule against expanding bullets is not (the US does not consider it customary IHL and, not being bound by any treaty provisions saying otherwise, is now buying and using hollowpoints).

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