So in the UN there are various types of missions they military force can do. You got your drone strikes, your direct intervention ( we fucking never do this one) , investigation (we do this alot, but we always do it way after the genocide has started and we already know its been happening and then classical and modern peacekeeping. Modern is peacekeeping with the intent of being a mediator, and a neutral ground for humanitarian aid, and to stop any serious war crimes from happening. You don't help either side.
Classic(Which we almost always use) is the same thing as before, except you don't attack ever. Self defense only. And you cannot give chase to enemy, and you can not stop a crime from happening(such as rounding people up to be shipped off).
So if you remember back in 12th grade history, you learned the UN has a council right , and on that council, 2 of them are communists(Whatever not anymore for Russia, but they still side with china most the time). So when the US says yes, they say no. You need full 100% voting for anything to pass, and they will NEVER pass direct intervention(we learned this from our many examples of genocides.
So go ahead to the Sudanese genocide. This bitch was the first genocide of the new millennia (*yay humanity*). UN says about 300,000 people have been killed by the government of Sudan, the largest tool was mass rape,including children ect. just overall fucked. So what is the UN doing? Nothing. China said no. So they are just having the world watch them (this is a *proven* genocide) all die and nothing happens. If the African Union tries to intervene, UN will condemn them. This aint new though.
