In the case of human rights violations and war crimes it is the U.N's responsibility to intervene. The U.N should not be leaving its responsibilities to the United States, time to step up. A proven war crime or human rights violation should suspend said countries membership in the EU thus negating their influence.
I have mix feelings about the United Nations (Especially allowing nations like Saudi Arabia lead the Human Rights council.) However another problem I have is that the United Nations acts more like a casual observer pointing to a major conflict and saying "Your bad for doing this" yet does absolutely nothing to prevent the disasters from growing. So them actually interfering in a conflict would actually being taking a stand against something like creating political pressure to put an end to a tyrannical regime, putting an "end" to a nations supporting endless occupations of territories without reason, cluster bombing civilians, propping up tyrannical puppet states, and taking away human rights.
They can because the United Nations is a group that supports one another. They are group that will always be there for each other when in need. They are there to help and guide the nation. They will always have the right according to the universal declaration of human rights