If a person or people violate the Geneva Conventions they are liable to be tried for war crimes. However, there is no permanent or objective body that has been created to enforce the conventions or to prosecute war crimes.
In order for someone to be tried for a war crime, his side generally has to lose the war. This is what happened in World War II. The Axis lost the war and so the Aliies set up tribunals to prosecute war crimes.
Recently, there have been special tribunals set up through the UN to prosecute alleged war crimes that ocurred in the former Yugoslavia and in Rwanda.