Edward Gibbon contributed much to science fiction; without him there could be no fall of the Galactic Empire. In chapter XXXVIII of The Decline and Fall, summing up his theme, he speculates on a historical what-if that has never been followed up on in SF, so far as I know, and probably won't be: What if there were another wave of barbarian invasions?
Post-apocalyptic fiction has plenty of goth/biker barbarians (and post-Gibbon history has shown that 'civilized' people can be plenty barbaric), but old style barbarian conquest of civilized lands has been relegated to the sword & sorcery shelves.
Gibbon agrees that barbarian conquest has had its day, and gives several reasons. The first and most basic is the Russians, who by Gibbon's time had pretty much solved the problem of the Eurasian steppe nomads at the source: The plough, the loom, and the forge are introduced on the banks of the Volga, the Oby, and the Lena; and the fiercest of the Tartar hordes have been taught to tremble and obey.
But a bit later in his list Gibbon provides the text of this post. In war, he says, the European forces are exercised by temperate and indecisive contests.
This is of interest to us because, bloodthirsty lot that we are, we want to write about blowing stuff up, especially but by no means limited to spaceships. Temperate! Indecisive! does not sound like the way to sell a war saga. But if the war is intemperate and decisive you won't have much of a saga, because it will end with Chapter Two:
There was a brilliant flash of light.
In fiction this only works once, and probably with real civilizations as well.
The wars of the 18th century were temperate and indecisive, or seemed to be, for fairly basic reasons of technology and economics. Serious warfare, as the 18th century knew it, was expensive stuff: paid regulars and keeping them paid and supplied; artillery; massive fortifications and ships of the line.
And the advantage lay heavily with the defense, tactically and strategically. Enough dirt and stone, or even half a meter of oak, would stop cannon balls. As for the strategic level, experience in the
17th and 18th 16th and 17th century showed what happened to armies that pushed beyond their supply lines: They devastated a province or two, then came down with dysentery and crapped themselves to death.
Thus 18th century war looked at the time like a cohesive system, inherent to an advanced proto industrial society, but this line of Gibbon is usually quoted for its irony value, because along came the French Revolution and Napoleon and all of that.
The modern view through the 20/20 hindsight rangefinder is that the French Revolution raised the stakes of warfare by harnessing the power of national mass mobilization. You could put far more troops in the field than the pre-1789 world had imagined, but only by arousing the mass passions of your population - at which point you were no longer really in control.
But there were a couple of military preconditions to all this. First (or so I gather), the French Revolution showed that a militia rabble could indeed defeat 18th century regulars, if they outnumbered the regulars massively enough and were fired up enough, and second, that with good sergeants you could turn that rabble into a decent army pretty quickly.
I vaguely recall something about an artillery officer from Corsica in this mix, too, and strategic mobility coming back into play, but my ignorance is profound here. Suffice it to say that the 18th century model of temperate and indecisive contests did not hold up.
Now let us imagine midfuture settings. The means of making war in a serious, great-power way are presumably still expensive. The means of simply nuking your enemy back to the stone age are available, and cheaper, but not the means to keep your enemy from nuking you back to the stone age.
Since nuking each other back to the stone age is against the general interest of all parties, could they avoid it by tacitly accepting temperate and indecisive contests, and scaling their objectives to suit?
Doing so by overt treaty, and making war by formalized rules, sounds vaguely tinselly and implausible to us. But the stakes of 18th century war had been implicitly limited by the same Treaty of Westphalia that has given the name 'Westphalian' to the whole concept of a state system and balance of power. Religion, which had made 16th and early
16th 17th century warfare so implacable, was more or less taken off the table as a reason for European states to go to war.
This agreement was possible because bitter experience had taught everyone that neither Protestantism nor Catholicism were going to go away, so there was no point fighting over them.
No such formal agreement might be needed in a future era, only a tacit understanding reinforced by the very powerful motives of elites toward self preservation. The fact that World War II happened does not negate this tendency; in 1939 not only was the atomic bomb still (literally) science fiction, but most of the offensive weapons and tactics of the war were still quasi-experimental and more or less untried. Now elites know what will happen to them, and it tends to concentrate minds.
Some classic SF scenarios lend themselves to temperate and indecisive contests, for example deep space trade wars. Trade warriors may be constrained on the one hand by the risk of burning their profit margin on military spending, and on the other hand by the disadvantages of vaporizing prospective customers.
At the other end of the spectrum, the Starship Troopers logic of racial wars of extermination pretty much points toward, well, wars of extermination. Pick your scenario and take your chances.
The image is a scene from the Battle of Minden, 1759.