Warcraft: Orcs and Humans
Warcraft: Orcs & Humans became by far the company's greatest success to that date, and for the first time made the company's finances secure.
Each faction's units and buildings have a counterpart within the other faction. Each unit is basically the same as its counterpart, with only slight variations. For example, Orc Spearmen do slightly more damage than Human Archers while having a slightly reduced range.
The success of Warcraft: Orcs & Humans motivated Blizzard to publish a sequel, Warcraft II: Tides of Darkness, and an expansion pack, Warcraft II: Beyond the Dark Portal.