WarCraft: Orcs & Humans
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From Diablo Wiki
WarCraft: Orcs & Humans was the first RTS game Blizzard Entertainment did, and was heavily inspired by Command & Conquer II. It opened up the world of Azeroth and is the seed with eventually became World of WarCraft.
Read more about the WarCraft series, and head over to WarCraftIII.net or WorldOfWar.net for more in-depth knowledge about the games.