I have played this game before, and there's nothing quite like it out there. You play the stereotypical villain in a science fiction franchise, that of the invading alien forces. What I really like about it is how the game keeps adding layers and layers as you progress.
You are given a set amount of troops per planet (the individual stages) to successfully conquer the planet with. What you get is ALL you're going to get, so you need to make the best use out of your existing troops as possible. Each planet in turn has different zones to them (you could think of them as different continents), and you need to assign troops to attack each individual one. If memory serves, everything relies on timing - when troops will arrive, when they can actually attack, how long it takes to drop into or return to the starships in orbit and more.
You begin the game very low on the totem pole, and you are only trusted by the empire to invade primitive planets - each planet has a technological level associated with it. But the higher the technological level, the more complicated things get. The cities are more defended, the soldiers are more organized. Once you hit the industrial levels of technology, radio communications become a thing and you can try to intimidate the planet's leaders to surrender after inflicting what you hope is enough damage.
After each planet is conquered, you are given a score reflecting the glory you've achieved, and then you're immediately sent off to the next planet. There's not really anything here in terms of 'downtime' between stages - it's immediately get a new set of troops and ships and off you go to the next conquest.
I didn't play this game for very long, but it's a game that's always stuck with me. As I said at the start, there's really not a whole lot like it out there, and I'd love to see it brought to modern systems. Who knows? Some game developer out there might get inspired by this. GOG, you all do amazing work. Please bring this one in!