In cases where states mistreat their citizens up to the extent of genocide, countries can decide to intervene even but it would be better to do so with UN approval.
The United Nations is the body that is responsible for checking the excesses of Countries. Countries that carry out certain actions that undermine the rights of citizens, extrajudicial killings and genocides are answerable to the UN.
Other powerful countries may decide to intervene in such situations but it is best that they do so under the approval of the UN. The reason is to avoid the resentment and terror that comes with such interventions
I believe the only time other countries have to intervene is if the UN fails to give their approvals in the face of such evils.
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