To my simpler mind cast back in not too distant times, it would seem that external intervention in internal strife is none too successful unless the  peace keepers are under a UN mandate. Civil wars such as in Korea, Vietnam and many independence struggles  resolve in several ways and external forces  have to leave eventually for it to happen.
Now that may be a wrong impression and an ignorant opinion so some further opinion would be appreciated from the better informed.
When the aggressor is from without as in the Great Wars then the will to fulfil  resolution of the conflict  is more clear cut.