In response to a wondering, which response grew too long to post as a replying comment.
It’s not about the gain, so much, any more than, say, the Cold War.
On one level, it’s because they’re both expansionary civilizations, the Republic and the Worlds. (I say the Worlds rather than the Empire because it’s the Worlds as a whole that they have an issue with; and, also, they’d have to fight a long way through a lot of people to reach the Empire, although they may hit up some distant Imperial ecumenical colonies.
…not that that stops some Worlds polities from complaining about the Empire fighting its war on their turf, the ungrateful sods.)
Even if the Republic destroyed the connections along the Borderline, and even if everyone in the Worlds agreed to let it alone (unlikely), the fundamental problem remains: they’re competing to absorb new territories (they’re less xenophobes than neophobes, to be fair to ’em), and they have to, or they’re going to end up surrounded and an isolated backwater.
Worse, and what really keeps Vonnie planners awake at night, is that as a centralizing/hierarchist civilization/polity, they’re starting to reach the limits of their control span and their grip on their outer territories is getting a little, ah, wobbly. The Worlds doesn’t suffer this disadvantage, because it’s not one civilization/polity, it’s lots of them, and a good few of them (such as the Empire, the Photonic Network, etc.) aren’t centralizing/hierarchist, they’re delocalized/cooperatist, and aren’t going to hit the limits of their control span any time soon, if ever. So to a certain extent, they’re gripped by some urgency on this point.
But even more importantly, they perceive each other as mutual ideological foes and existential threats. They represent contending visions of the future. And, indeed, most of the differences they have arise from that split. They have lots of differences, some familiar, some less so:
- planned economy vs. free-for-all
- ephemeralism vs. immortality
- computers as tools vs. computers as companions
- naturalism vs. augmentation
- restricted technology/warts-and-all people vs. superempowering technology/better people
- control vs. trust
But it all boils down to that the Republic is, at its base, conservative/preservationist. (Note: not as in current politics; insofar as kalatri domestic politics can be mapped onto anything Earthwise at all, they’d be pretty seriously progressive and humanist [well, kalatrist].) The future they offer is based in their ideals of preserving sophont nature and its supreme inherent value – the kalatriness of kalatri, the humanness of humanity, all of that; it’s a universe by, for, and of natural species “baselines” for ever and ever, amen, or at the most what “natural” evolution makes of them. (At least until the cold and the dark of the post-stellar era comes along and wipes ’em all out.)
The future the Worlds have to offer – no, let’s say the future the Empire has to offer —
Well. It’s principally the Empire that are the foremost exemplars of this, but the Worlds in general are the home of all sorts of social and technological experimentation that offends the Vonnies’ principles. They dislike the Equality Concord, etc., too, almost as much.
Anyway, that future is vaguer, because it involves any number of people chasing down independent dreams just as hard as they can, but the Empire and other similar polities in the Worlds are charging hell-for-leather for singularity after singularity, pausing briefly at uebermensch on their way to postmensch, with – especially in the Empire – all thought of constraint stripped away and considerations of eldrae, etc., “nature” thoroughly discarded in favor of their Vision Of What Ought To Be. But what we, and they, can be certain of is that it doesn’t belong to the baselines. It’s going to belong to ancient immortal consciousnesses enshrined in organic crystal computers and multilayered collective consciousnesses and Vingean Powers and the long, slow, massive thoughts of an entire galaxy turned into a constellation of computronium moon-brains that may, with some small parts of itself, sometimes remember individual incarnation and play at flesh once more. Unaugmented baselines – well, they just can’t compete. It’s a future of gods and angels, not men.
And these visions are intrinsically incompatible. Thus:
The Empire has to stop the regressive tyrants of the Republic from condemning the galaxy, and possibly the universe, to a future of such petty mediocrity. And the rest of the Worlds, squabbling and disagreeable as they may be, don’t want that future either.
The Republic has to stop the madmen next door, and especially the Empire, from turning all the “real” people in the galaxy into monsters more machine than man and/or the reservation pets of unleashed technological demon-gods.
(And they’ve got to do it now before they’re too strong to stop with the Republic’s weight of metal. Whatever the official public line might be, the Vonnie strategic planners are well aware that 99% of the time, baselines fighting a mature postsophont intelligence can be summed up as “You Lose”.)