Sylvanas said she wanted to take over the world by any means necessary and acted the part. The Horde backed her in their vast majority under the assumption that she was doing the above for the reasons she said. She then revealed that she wasn't actually aiming to take over the world and was instead going to fuck them over, then insulted them and fled. If even her cover story is objectionable and the game assures it was in one interminable cinematic after the next and the Horde supported her on the basis of that objectionable cover story, then it is the Horde's war. Especially since the second it ceased to be about that ground publicly and became about Sylvanas's goals, it ended.