> Any combatant with the necessary wealth and technology to develop any space-combat capability is going to have significant terrestrial interests
Imagine a colony that long ago severed its ties with Earth (or whatever other terrestrial body they started from). Living off comets and asteroids seems entirely feasible and it's a much harder to attack/easier to defend set up.
If the outer solar system dwellers become self-sufficient, they would have no need to go down the gravity well of the Sun to have commerce with earthlings. The further out those colonies are, the lesser their contact with Earth will be. If they live off the Oort cloud, the would be about one light year away from Earth and it's very hard to imagine a shared culture with such a communication lag.
Why, exactly, would people dwell in the Oort cloud? The idea of having an isolated, self-sufficient colony in the Oort cloud seems like an even poorer version of North Korea, except you'd have to ship all the people out there.
There's no good analogy with colonialism on Earth, because most of that involved getting very very rich by trading with the mother country, and if there's anything to be won, economically, from the moon or the astroid belt or even the Oort cloud, it'll be from Earth trade. As far out as the Oort cloud, you'd be better off sending robots than human colonists for the very same reasons you point out.
What if the people want the exile? Provided fabrication technology is advanced enough to provide for material comfort, what is the difference between living on a planetary surface or living on a very comfortable habitat? What if you don't like Earth laws?
That same fabrication technology would negate the traditional colonial model. There would be very little need to move physical goods between worlds.
Imagine having a million times more space and material resources than you currently have on Earth now. While the mass on the Oort cloud is a fraction of the mass of the Earth, it's still a lot of stuff.
Alright, let's go with that scenario for the sake of argument. What would be the casus belli?
If it's something like rare elements available only in the Oort cloud, the traditional colonial model would apply and the overwhelming interest in the Oort cloud would be in those very rare elements and not as some sort of refuge. Your Oort cloud pilgrims would have to be fiercely protective of their rare elements and to have gotten there first before the rare-element prospectors. The Oort cloud would have to be not big enough to satisfy both groups of people, and the Oort pilgrims would have to be stubborn enough not to move their replicator-colony to the Kuiper belt or elsewhere.
At that point, you can start to reasonably speculate: you have clear objectives on both sides at the very least. It becomes very hard to predict what would happen because we've stipulated so many counterfactuals that everything we know would be obsolete. Logistics, which has dictated almost every war in human history, would look totally different due to the fabrication technology, though limits to propulsion technology and the vast distances involved would cause other issues. The Oort pilgrims would have the advantage of locality--they could see and respond to any movements from the inner solar system within months while delaying or hiding their defensive actions until the last minute. But the fabrication technology would render pretty much anything expendable, and it might be feasible to turn maybe a few hundred Kuiper belt objects into a swarm of self-guiding missiles or something while the Oort pilgrims wouldn't have enough raw materials to really hurt, say, Earth.
Imagine a colony that long ago severed its ties with Earth (or whatever other terrestrial body they started from). Living off comets and asteroids seems entirely feasible and it's a much harder to attack/easier to defend set up.