So, I have to say it:
I’m tired of the inscrutable, evil-for-no-reason-we-understand and essentially undefeatable alien trope.
I love when it’s subverted. When, for example, an outsider to the society comes in and tells the aliens to stop fighting. I suppose I subverted it a bit in Transpecial, although it wasn’t conscious.
But there are other ways to write your six book series set in the same war.
Who’s Fault Is It?
TV Tropes calls this trope “Aliens Are Bastards,” but I feel they miss some examples. For example, they miss Fred Saberhagen’s infamous Berserkers. Although, at least they don’t blame it all on Starship Troopers (the Bugs are two-dimensional, but hardly impossible to defeat and their motivations, while simple, are understandable. They just want the same real estate we do).
Then, of course, there’s Alien, still my favorite horror movie of all time. The xenomorphs are pretty nasty, but it is supposed to be horror. I think that makes a difference.
I’d say the most egregious examples are War of the Worlds and Independence Day. The latter is very much a homage to the former. Instead of the aliens being defeated by a biological virus, they apparently run on Windows. But it’s the same thing. They show up, they attack for no reason (in fact, Independence Day is worse; the Martians do have a reason).
But I’d say it’s a military science fiction trope, because it lets you focus on the nitty gritty of the war without ever having to worry about why there’s a war going on.
What brought this up was I recently read a book. I won’t name and shame, but the aliens were evil for the sake of being evil and took humans as slaves for reasons never properly explained in the book. And read DNA using gamma radiation lidar…ahem.
How to Avoid It
Start by giving your aliens a motivation. It doesn’t have to be very complicated. Maybe the aliens and humans both want to colonize the same star system. Maybe it’s about resources. Maybe it’s all One Big Misunderstanding like the Minbari War in Babylon 5.
Then, remember that your aliens have a goal here. And they are people. War is what happens when diplomacy either breaks down or is not attempted. (And diplomacy can be a fascinating story too). If it’s not attempted, why? Language barrier? Technology barrier?
Now, think about how long you need your war to last. Bear in mind that you can always write your long series by focusing in on small parts of the war, by setting books on different planets during different actions.
Set obstacles in the way of peace that aren’t “They’re bastards and just don’t want it.” Maybe the obstacles are hawkish humans who don’t want peace. Maybe space is just too big and actions take months or years because of communications delays.
Maybe there’s that one sticking point the diplomats can’t agree on. It’s not that hard to make a space war last years.
But you really want indefinite war?
First, ask yourself why it’s necessary. Second, you can always introduce multiple enemies. You can ally with the first set of aliens and then have something worse show up.
Just don’t resort to “They’re evil so we have to kill all of them.” It’s overdone and it goes into the “evil by birth” trope that makes many people uncomfortable (especially PoC and Jews, natch).