system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 What makes countries follow Geneva Convention in war? Just because an action is deemed illegal, what’s the incentive for countries in war to abide by it?
gpt
Because they want their opponents to also abide by the rules. While there is a slight advantage to, say, not looking after enemy POWs properly, there is a huge disadvantage to not having your own POWs not looked after by the enemy properly. So this provides an incentive for all combatants to abide by the rules regarding POWs (or at least, *be seen to* be abiding by those rules). 

This is also why verification organisations such as the Red Cross are so important.