In many cases the Geneva conventions are enforced within the armed forces of the signatories. Those that have agreed to these treaties have also agreed to enforce them as laws within their militaries.
In the United States, for example, doing things that would violate the Geneva Conventions generally violate the Uniform Code of Military Justice. Those that do so are tried and punished under the UCMJ.
Of course, sometimes there are disputes. In general if a sovereign nation disputes a treaty violation there is no way to enforce it beyond what pressure the international community is willing to bring.
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