Conquest Frontier Wars is an awesome scifi RTS and is still one of my favourites.
You control a fleet of ships in either the Terran, Mantis or Celareon race. The campaign is Terran only but you do get chances to control others. Speaking of the campaign, its a long one. There a sixteen missions but each mission is long and hard and requires strategy in order to beat. It starts off pretty easy, introducing you to the mechanics, such as harvesting ore and gas, and building structures on planets and building fleets. The difficulty builds up quite quickly, and the missions become quite hard later on.
Unfortunately, there is no one left playing multiplayer, so that particular aspect of the game I cannot review.
So all you can do other than the campaign is single player custom games. Fortunately, these provide much challenge and fun. The matches start off slowly, you build up your base and you explore the neighbouring systems. The game does a good job of encouraging you to use scouts, and you should definitely use them because getting caught in a battle in unknown coordinates can make it much harder to escape. The missions then gradually increase in tactical intensity, and most battle have a pretty close finish. One great thing about this game is that it is quite challenging on high difficulties, and really requires strategy. In fact, due to having to use supply ships to keep weapons stocked up, if you just make the biggest fleet you can and send them in all at once, you will probably still use.
Conquest Frontier Wars is an awesome RTS which requires some real brains to outsmart the opponent on high difficulties, while providing an enjoyable experience all round. The games can also last a long times, one of my custom games lasted around 5 hours. I just wish they made the sequel, because this is one game you've probably never heard of, but is absolutely awesome.