TheBoost said:

Thinkin' about the original "Star Wars," there's no reason to assume that there ARE any aliens in the Empire or the Rebellion. For all we know it's the Human Empire, the government for human beings, one of many governments in the galaxy far far away, and not of any particular concern to aliens, who all have their own governments with their own problems.

No, that idea is thrown out by the film's opening crawl, which tells you that it's the "galactic empire", implying it runs the whole galaxy. Which doesn't imply a situation in which it's just one of many governments in the galaxy.

Sluggo said:

I was going to argue this, but after racking my brain for a few minutes, I can't think of any threat to alien species at all.

Stormtroopers killing Jawas comes to mind. It's pretty clear the imperials are going to kick around anybody who gets in their way, human or alien and since they're the "galactic empire" that runs the galaxy, that's going to include the aliens that are in the galaxy.