I changed this to "science fiction" as space opera seems the wrong categorization to me, and as the article on space opera describes that genre as emphasizing romantic adventure, interstellar travel, and space battles, none of which are main elements in Starcraft (though one must assume that interstellar travel takes place, and there are a few space-station battles).  If anyone can help me out with a better subgenre, I'd appreciate it, but the assignment of the game to space opera seems highly specious  (though perhaps there's something I'm missing?).