> all that matters if I have enough military power to keep them in the saddle
That's pretty much how most declarations of independence have worked historically. But even if you have enough military power, how much blood are you willing to shed if a full blown civil war breaks out? If there are civilian deaths, it's likely that there would be condemnation and support for the separatists from the international community. What matters is how badly people want their independence and how far they are willing to go for it.
Please see the parent comment. My opinion here is in regards to "what matters to what happened 1000 years ago? what matters is what they want now!" - by making a point that what happened years ago does indeed have some value.
That's pretty much how most declarations of independence have worked historically. But even if you have enough military power, how much blood are you willing to shed if a full blown civil war breaks out? If there are civilian deaths, it's likely that there would be condemnation and support for the separatists from the international community. What matters is how badly people want their independence and how far they are willing to go for it.