There are lots of Geneva Conventions, cos it's the European home of the UN, but the one everyone talks about is the one about war crimes. The Geneva Convention (No. IV) entered into force October 21, 1950. The Convention was immediately signed by most of major powers, including the United States and the Soviet Union. The United States ratified the Convention on February 2, 1956. Many of the signatories (including the United States) made reservations at the time of signing and/or ratification. The Convention has gained widespread acceptance and it is likely that most of its principles now constitute customary international law; so non-signatories may therefore be legally obligated to abide by
The Convention is essentially a response to German and Japanese actions in World War II (though it should be noted that the Convention's principles were violated by all the major powers in that conflict, including those states responsible for the Convention). The Geneva Convention's general aim is to create rules of war which provide basic protection to
non-combatants (as well as captured military personnel).
It is a convention, but not a conventional one like in domestic politics. Breach of it is actionable, but it is full of weasel words and in 150 languages. The pertinent articles of the Convention apply only to actions of states (by direct policy or by individual acts by officials of the state, including military personnel) which take place during a declared war or an armed conflict, even if a state of war is not recognized by one of them. (Note that this language is ambiguous. The Convention might not apply if, despite an ongoing armed conflict, neither state officially recognizes a state of war.) The articles apply to any occupation, even if that occupation does not meet with armed resistance. The articles do not bind signatories in their actions against non-signatories, though as noted above the Conventions may have achieved customary international law status. The question would then become whether the Convention's reciprocity trigger is also customary.
Article 146 establishes the signatories' obligations with respect to enforcement and prevention. For violations of Article 147 ("grave breaches") the contracting states are obligated to legislatively provide for penal sanction against violators and to search for and (fairly) try alleged violators. For all other violations of the Convention signatories must whatever measures are necessary to suppress such acts. These measures are not detailed. The penalties, if any, for failure to fulfill these affirmative obligations are not discussed.
Deliberate violation of the Convention by the State is grounds for action against the State by the Security Council. Deliberate violation of the convention by individuals should be dealt with by the law of the country or occupying army. The UN can throw together a Nuremberg-type war crimes tribunal, or throw vague bits of human rights laws at the perpetrators. However, the convention itself has no enforcement provisions whatsoever - states must merely deal with "grave breaches" through "international agreements".
So, the crimes are vague, can be excused by military necessity and matters of opinion and evidence. Enforcement is even trickier, every single country has violated it many times and won't stop. The USA is probably the worst offender as it has invaded dozens of countries over the years, but may be the most humane insofar as the more visible atrocities are not considered by the yanks to be militarily useful. The USA has used the Security Council veto more than anyone else, and has never, ever, submitted to a single UN guideline, rule, law, court or UN motion relating to its use of miltary force. It does not submit to international law, it uses international law and its lawyers and armies as a tool of diplomacy.
There is a really good, tough protocol from 1978 which would add international teeth to the Geneva convention and get rid of the loophole in the convention that allows collateral damage. Alas, it's not international law because only Ghana, Libya and El Salvador have signed it.
There is law, and there is guns.