Pluto Campaign -> Hydora Campaign -> Tophet Campaign -> Tesca Campaign -> Zephyr Campaign -> Klendathu Campaign -> Homefront Campaign
Starts off with a skirmish/extermination on Pluto, before they find that the bugs came in from elsewhere, after which they tail a transport and discover the pit-stop on Hydora. Tophet is sort of a proxy-war for the Bugs, introducing the infested Skinnies as Bug 'allies', while Tesca depicts the troopers exploring Dagobah and kicking Yoda's ass. Zephyr is an "asteroid" the Roughnecks crash into en route to a mission, before making a rather shocking discovery. Klendathu covers, obviously, the invasion of the Bug homeworld and discovery of a Queen Bug. And finally, war hits the homefront when the Bugs attack Earth itself.