In stereotypical fantasy, there is normally some sort of revolution in a uni-polar world where the rebels have to fight the evil empire. (This is not always true but its basically what happens) The "empire" either is the only country that exists or sits alone on a continent etc... These are semi-plausible excuses since (I would imagine) in the old times, when the Roman/ Chinese empires had a civil war/ revolutions, they were mostly self contained w/ Romans fighting each other or Chinese fighting each other. While there were foreign/ barbarian mercenaries and such, its not like the leader of the Germans sent in an expeditionary force to provide humanitarian aid or something. Again the no outsiders/ foreigners rule does not always hold true as after the fall of the Ming, the Manchu (who were still apart of the Ming Dynasty but were considered "outsiders") took advantage of the situation to take power.
In our modern world, whenever there is conflict, its used as a proxy war by other powers to push some sort of goal. The powers funnel arms, money and even sometimes troops to accomplish their goal. The war in Yemen has KSA + USA vs Iran. The war in the Ukrainian has western countries pumping in weapons and money and even before that the war in the Donbas region had Ukraine and the West vs the rebel with support of Russia. Syria had Russian + Iran vs the US and some others. Even in tiny countries this happens see https://en.wikipedia.org/wiki/2021_Solomon_Islands_unrest where China + Australia intervened. Would it even be possible in our modern interconnected world for there to be just a civil war in a populated country (think US, China, India, Brazil etc...) without its rivals swooping in to aid a favored side? Would it be possible in the future via some invented technologies?
thanks