system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What laws (international presumably) if any has Russia broken in invading Crimea?
gpt
It is against the rules of the United Nations for any member country to invade the territory of another unless the Security Council has explicitly permitted such an action. That's one of the reasons that the 2002 allied invasion of Iraq was controversial -- depending on which legal interpretation you preferred it was by no means certain that the Security Council had permitted an invasion.

In the case of Russia and Ukraine it's far more clear-cut -- the Security Council has not agreed to any action by Russia in that respect.

The big problem with enforcing international law is that there is no recognised global police force. Even if a country has obviously broken the rules, unless other countries are prepared to take action (economic, political, or military) there are few if any repercussions.