This sort of question is best answered with the logical class, reductio ad absurdem: what would happen if there wasn't a single body of law which governed actions in the UN building? read more
> Though it is in New York City, and part of the United States, the land used by the United Nations Headquarters is under the administration of the United Nations, while also being subject to most local, state, and federal laws. read more
At the moment, international criminal law only covers the most heinous crimes of genocide, war crimes, crimes against humanity and crimes of aggression. Those laws are too narrow to address many of the abuses committed by peacekeepers. And international humanitarian law governs only parties to armed conflicts. read more
3. This would require a police force from each nation, or alternately it would require that the UN security police would have to be aware of every law in every nation of the UN. I could go on, but it's facially absurd to have a plurality of legal systems at play in the UN building. read more