Could it be Overlord, aka Supremacy?
The goal of Supremacy is to create and protect a network of planetary colonies and defeat a computer adversary who is trying to do the same. There are four skill levels, each represented by an enemy race, and each featuring a progressively stronger opponent. The more advanced a system is, the more freedom a player has when purchasing spacecraft. Higher skill levels also result in different numbers of planets in each system.
The game is controlled using a mouse pointer, and important information is displayed in a message box at the bottom of most screens. Following the introduction screen, the player chooses which planetary system to enter. Planetary systems differ in the number of planets they contain, the types of spacecraft and equipment that can be purchased, and the strength and aggressiveness of the artificial intelligence. The first system contains eight worlds and allows access to only the most basic equipment. The second system has sixteen planets and slightly better equipment. The remaining systems increase in size and strength similarly.
The player and his opponent both begin with control over a single colonized planet in the chosen system. All other planets in between are uninhabited and up for grabs. The player then must set up and maintain thriving colonies on as many planets as possible, and build up an industry and military strong enough to fend off the opponent. Because of the randomness of a system's initial make-up, the order in which planets are colonized has a great subsequent effect upon gameplay.