Moving back to Blizzcon 2017, here are Alex Afrasiabi’s remarks from when they were still keeping some mystery about the order of Teldrassil and Undercity.
https://warcraft.blizzplanet.com/blog/comments/blizzcon-2017-world-warcraft-battle-azeroth-panel-transcript
Alex: We are here today to talk about the Battle for Azeroth and prepare you for the coming war. Right now, we have all seen that incredible cinematic. We are from the get-go: battle lines are drawn, and what I like best about that cinematic (and it does so many things, so well) is that it reinforces the foundations of Warcraft; because whether we are fighting the Burning Legion, the Scourge, Old gods, Dragon Aspects, Elemental Lords, or yes… timetraveling Orcs, our hearts and minds are with our factions. We do what we do in this worlds, first and foremost, for the HORDE! … and for the ALLIANCE! — and that makes us the champions of Azeroth. The greatest hope the world has ever known, and also its biggest threat. But this is WoW. We don’t just get to watch awesome cinematics. We get to live them. Because to get to the Battle for Azeroth, you are going to have to go through the battle for Lordaeron.
Like the Broken Shore, we will pick up where the cinematic left off; and what goes down that day will change Warcraft history forever. Now some of you might be asking: Hey, Alex. Why would the Alliance attack the Undercity? It is a good question. It is a good question. Here is a better question: why would the Horde burn down Teldrassil?
Did Teldrassil fall first, and lead to the attack of Undercity; or was the burning of Teldrassil a response to the bold Alliance attack on Lordaeron? You will find out. Think about this: the stage is set for an Alliance-controlled Eastern Kingdoms, and a Horde-controlled Kalimdor with one great big sea separating the two continents — which leads us to our search for allies. In our quest for victory, we will seek out the Kul’Tirans as the Alliance, and the Zandalari as the Horde. Coincidentally, both of these nations have extremely powerful navies, which we are going to need to span that great sea. Let me leave you with this thought: no matter who fired the first shot, we are in an all-out war. It is our duty as Alliance, as Horde, to defend our people, our families, and our homes. This is the Battle for Azeroth.
So to recap: the Alliance and the Horde are in an all-out war. All hatreds have reignited. It is both factions strike at each other’s hearts. Now with the world divided, we seek out lost territories, and new powerful resources; and with no external threats, we turn our fury to each other. We fight for our faction’s survival, and our place in this world.