Actualy the warhammer 40k universe is used by a lot of games on lots of platforms but it started out before any 3d games back in the maybe the late 70s but i think it was the early 80s as a "tabletop game" where people used real life figure things on sets or something like and it still exists today. The person or people that thought the universe up had been working on the universe for a long time so its incredibly complete so a lot of video games and books are based on it.
I can think of 5 games that have been released for pc alone then 1 expansion for one of those games and 2 that are in development that all use the warhammer 40k universe!!! The warhammer 40k: Dawn of war games are made by Relic and are a lot like Company of Heroes which is also made by Relic. I actually prefer both of them over world in conflict as far game play goes though the Dawn of war story is pretty weird until you get into the game more. Why dont you try the demos for the games they are completely free?
Warhammer 40k: Dawn of War Demo (original from 05 or 04 but don't be fooled by its age Its still awesome!!!)
http://www.gamershell.com/download_6943.shtml
Warhammer 40K: Dawn of War 2 (I dont think its even a year old)
http://www.filefront.com/13605067/Warhammer-40000-Dawn-of-War-2-Demo/
World in Conflict Demo (not as good as the whole game. And World in Conflict has a good story line that you cant see in the demo)
http://www.filefront.com/8391048/World-in-Conflict-Demo/
Company of Heroes Demo (Its a world war 2 rts but its got great game play and a few good looking mods in development including "Eastern Front" that seems to be up to the standards of the actual game if not higher!!!)
http://www.filefront.com/5473388/Company-of-Heroes-Single-Player-Demo/
That should get you started but try as many as you can demos usualy give you a good idea of the gameplay.
A few others that i can think of that should have demos are.
Supreme commander (not sure how many there are)
Starcraft (not sure how many there are though its already been mentioned a lot by hellokitty[hk] so ask him/her/hobo)
Red alert (suposed to be pretty good)
Command and Conquer (i think red alert is a spin off of these)
Wait i know here is a list of them all!
http://rakrent.com/rtsc/
One more thing There are some good mods for some of these games that can make you want to replay them a lot more so keep an eye out.
Edit: I have dawn of war dark crusade it is not turn based but in the story mod you do have to take over an entire planet and in between matches its sorta turn based, but only in that you and your enemies choose which territory to try and take from each other or defend in a normal rts match.