I think I've seen enough Star Trek episodes to have a phD in this subject...
It's simple. Here's a scenerio: On Planet Droog, there is one large continent. Resources are spread throughout, but there are certain areas that have a denser concentration of certain resources than others. As the Droogians evolve, they naturally spread and populate the entire continent.
After a while, their tribes become villages, villages towns, towns become cities, and throughout all this, they develop nations. Now because no one area has ALL the resources an advanced civilization need, they need to work out a way to get other resources they don't have.
Now lets say that on Planet Droog, there used to be a LOT of very large predators. The Droogians evolved with sharp teeth and claws and a fierce group mentality, because they needed all of these to defend themselves against the huge predators.
So we have this species which evolved fighting for survival. Now Nation Droogwin wants some resources from Nation Droogawesome. Instead of offering a trade, they decide to just take it.
So it gets to the point where there are only two nations left, Droogwin and Droogfight. Droogwin has a secret program to develop a spaceship capable of firing weapons from space. They do. They win.
Now Droogwin is the single unified nation of Droog, and they decide to explore a nearby planet for more resources. They find a primitive life form there, proving that they aren't alone in the Galaxy. Because they are hostile, they assume all other races will be hostile. In the next 50 years they make huge advances in space weapons and propulsion technology, and because their entire planet is able to work together under one nation, they are able to settle all 7 planets in their solar system in that time.
Fast forward another 150 years, the Droog have discovered and enslaved two other species in new solar systems, when they discover a little planet called Earth...