Before there was Total War, Command & Conquer, or StarCraft, there was Dune 2. Developed by Westwood Studios in 1992 and originally released for MS-DOS (though most people remember the Genesis version, one year later), it's widely considered the first game to create the RTS mold that is still in use today.
Actually, it's technically the second; TechnoSoft's Herzog Zwei, which also came out for the Genesis, predates Dune 2 by about a couple of years.
Still, Dune 2's influence cannot be understated, and those who are interested in going back in time might be interested in knowing that an open source remake from a few years back, called OpenDune, has just been ported to HTML 5.
Meaning, by opening this link in another tab, you can experience gaming history right this second, and for totally free. And for your RTS diehards out there, be prepared to marvel how far the genre has come in the past twenty years.