Games you remember playing but can't remember the name

[DanielRand47]

One PC game I remember playing had a futuristic space theme to it where you invaded an alien planet as a space marine sent from Earth. You ended up getting seperated from your squad and had to make your way to the heart of the alien city and kill their leader. But before that you had to destroy some big gun that the aliens had. It was obviously a FPS game. I'll remember the title when it is no longer relevant. Tends to be my luck.