Depends on the nature of the alien, if it's means of conquest is simply to anihilate a planet and then to occupy rape it's resources and move on like insects, then easilly possible but would require a large base of conquerable worlds or a way to become sustainable given enough fodder. To hold and to control a planets populace, now that is much more difficult, in space empires just as in land empires it's incredibly complicated, but that's not to say an alien might have more luck than us.
Imagine if an alien could alter the genetic structure of all sentient life on a planet, by mutating it to be something simillar to it's own form by using mutagens that would totally infect the whole ecosystem of a planet, also with this comes an understanding that to really grow it must take the best parts of a race but control the beings, ruthlessly supplanting individual will but keeping traits that help it to grow: in essence, you become the alien and somewhat keep the traits that benefit your masters. It would be easy to control a race if you had mutated them into a form where they were alien enough to obey completely.
Without such unimaginably alien means though they would have little more success than the empires of Earth and constant war would mean an ascendence of one race over another throughout history much as it is on Earth.
Another means is through a host system or a mutual symbiosis, but this would take free will of the people you are controlling, in a large stretch of races, it would become easy to overthrow such a race if you were technologically simillar, as they relly too much on a host.
Another way is to form a loose confederation of like minded worlds into an Empire, as far as Earth goes this is the most likely to be successful in the long term, but it does need some serious diplomacy skills and again it may well lead to overthrow and constant changing of figureheads.
Of course if your alien had powers of the mind that far exceeded human abilities it may well use these to control it, but this would be difficult to maintain unless the race had some sort of hive mind, like the Borg.
I can only see complete anihilation of a species as viable for conquest in the long term. Slaves rebel.
Perfection said:
This is much to broad a question to have a coherant answer. Who is the attacker? Who is the defender? What are their technical capabilities? What are the motivations behind each side? What is the planet like? You'd have to answer these questions before you could begin to realisticly look at what is possible or not.
Use your imagination I guess?
