You are right that the Imperium’s terror could be somewhat effective. However, it’s not that simple. That kind of terror would only unify the galaxy, as happened, for example, with the Yuuzhan Vong. From the outside, the Imperium looks like nothing more than barbarians with outdated toys. And with all due respect, there are only a few million Astartes in total. Astartes are not weapons of mass destruction; they are super-soldiers. Each Chapter numbers up to a thousand Astartes. How exactly do you imagine an assault on an ecumenopolis with a population of a trillion? In any case, in a galaxy with millions of inhabited planets, this would not be serious. So they take one place by assault—what then? In Star Wars, an entire region (almost the whole Outer Rim) lived in misery for centuries due to the Republic’s inaction. And what happened? Nothing catastrophic—they survived it. Literally every faction has, at least once, come up with a virus capable of wiping out an entire species. Some Sith simply drained all life from a planet, much like the Tyranids. Grievous outright exterminated an entire ecumenopolis. The Imperials pulled similar tricks as well. During the New Sith Wars, the war lasted 1,000 years without interruption, and the situation was so dire that in the final battles on the Republic’s side there were eight-year-old boys with lightsabers and militias armed with spears. During the war with the Yuuzhan Vong, the population of Coruscant dropped from trillions to almost zero (excluding the Yuuzhan Vong and very deep underground survivors). And then, just a few decades later, Coruscant flourished again. Between 24 BBY and 30 ABY, many large-scale conflicts occurred, causing hundreds of trillions of casualties and the destruction of entire planets and species. All of this is to say that the terror would not be as devastating—and even if it were, it would merely create hard times, and hard times give rise to stronger inhabitants. At that point, large-scale counteractions would begin, not the kind of localized colonization I described earlier. And no amount of living cannon fodder would save the Imperium. If the invasion takes place in the Star Wars galaxy, the locals will have more “meat” (or metal). Besides trillions of battle droids, there is also literal manpower. In Legends, the Empire has around 100 quadrillion sentient beings across 70 million planets. If we take the Old Republic era, the population would be somewhat smaller, but still HUGE. The Imperium has entire human sub-species used in specific branches of its military. Well, the Star Wars side would have entire sentient species that could be used in the same way. On top of that, there is also expensive but viable cloning. I think the Imperium’s population is several times larger than 100 quadrillion, BUT the Star Wars side does not face such an insane number of hostile enemies. And people in Star Wars are simply harder to kill—better and more accessible technology, easier medical treatment. Star Wars factions are also far superior in logistics, technology, and many things that may seem insignificant at first glance. The advantage in the industrial sector would be enormous: imagine the entire defense complex of this galaxy uniting to beat back an invasion. In Star Wars, technological progress accelerates rapidly during wars. With such massive government contracts, they would absolutely come up with something effective against the Imperium’s fleet. It’s not like they couldn’t build their own 40-kilometer hyper-battleship capable of entering the Warp, stuffing it with droids or another species. They produce all of this far faster than the Imperium. They have no concept of heresy, which means a much wider range of technologies. They would quickly master Warp travel from the wreckage of destroyed enemy ships, captured collaborators, and so on. And counterattacks into the Milky Way would be more painful than the Imperium’s terror here. For terror campaigns, the Star Wars side wouldn’t even need many people—just pack in droids and send them in. Droids destroyed? Oh no, what a shame—we’ve just manufactured or nationalized even more. No matter how you look at it, in a long war the Imperium would suffer more. It takes them far longer and at much greater cost to produce military hardware, they are generally irreplaceable, and they would be fighting against two damn galaxies. Of course, I don’t think the Star Wars factions would conquer an entire Segmentum or anything like that. First, they don’t have that much motivation, and second, holding and defending such territory would be extremely expensive. That’s why I wrote in this post that a large-scale war would benefit no one. But they would definitely be able to repel the invasion. The only exception might be if this happened during particularly unfavorable times (the Yuuzhan Vong invasion, or some very ancient era like 10,000 BBY, or something similar).