They're only useful after the shooting has stopped
Everybody knows this: The modern UN comes into play only when the two parties have agreed to a cease fire and someone needs to mind a buffer zone. The UN did play a major role in the Korean War (it provided the legal basis for the US to stop North Korea / China's aggression). Also (of course) the victorious allies of World War II called themselves the "United Nations" shortly before the defeat of the Axis.
Gary