that would fall on Intervention which is considered illegal in most cases unless its by invitation, request or Humanitarian intervention which i think it falls on since is

the use or threat of military force by a state (or states) across borders with the intent of ending severe and widespread human rights violations in a state which has not given permission for the use of force.

you would need the UN via the ICJ would have to approve South Africa's case of Genocide in order to have an authorized intervension but other countries have use the HI without the support of the UN Security council like during the NATO intervention in Kosovo