First 3D strategy videogame

Warhammer: Shadow of the Horned Rat (Mindscape, 1995) featured true 3D terrain, although Total Annihiliation (Cavedog Entertainment, 1997) was the first game to feature both fully 3D terrain and 3D units. More impressively, however, Homeworld (Relic, 1999) featured a full 3D space simulation in which you could control your ships, allowing you to establish proper 3D formations and allowing for tactics employing the full breadth (it took the Cataclysm expansion pack to make it fully playable though).