So the three alliances all have their own reasons why they feel like they should be ruling Cyrodiil, but the fundamental reason is that the Empire and Cyrodiil and the family of nobles – the Tharns – that are ruling it are weak, and the alliances are strong.
So the Empire is weak, but their leaders are cunning. The current occupant of the Ruby Throne in the Imperial City is Empress Regent: Clivia Tharn, and her father is actually the Chancellor of the Elder Council – his name is Abnur Tharn – and he’s a very old but powerful and really wily Battlemage, and his family have been secret Daedra worshippers for generations. So when he sees the threat of the three alliances around the periphery their inevitable interest Cyrodiil, he turns to his family’s old allies: the Daedra, and Molag Bal’s main agent on Tamriel, the Necromancer Lord Mannimarco. As the alliances become aware of Abnur Tharn’s Daedric connections, that becomes just one more powerful incentive to go into Cyrodiil and take him out.
The alliances are very different and they have different reasons and they have different justifications for why they are going to be marching in the center of the continent, and this goes back to the individual leadership of the three alliances and their motivations for why they’re doing this, in many ways, reflects the values of the alliances themselves.
The reason why The Daggerfall Covenant feels that they must prosecute this war really goes back to history for them. The leader of the Daggerfall Covenant is High King Emeric, a Breton King. He knows the history of the Empire well and he understands that the ruler must be strong to be able bring peace and prosperity into every village. Emeric sees that current efforts are not enough. Humans are failing and Elves are rising. So the Covenant must take up the torch and take Ruby Throne.
The Ebonheart Pact is more attuned to the mystical side of things. When they become aware that there’s a great threat to Nirn, they decide that they’re going to have to mobilize to confront it. Almalexia, one of Ebonheart Pact leaders and Tribunal gods, found that Molag Bal has got some kind of great magical existential threat to Nirn. Now, having the Empire working with Molag Bal and Mannimarco absolutely cannot be tolerated, so the Ebonheart Pact decides they are going to have to muster their forces and march on Cyrodiil.
The third alliance, The Aldmeri Dominion, is led by Queen Ayrenn of the High Elves, and her reasons for pushing this war into Tamriel are not only geopolitical. She has some personal reasons to be involved into the conflict. Ayrenn had been an adventurer and traveler in Tamriel before she’d assumed the throne. She had suffered personally at the hands of Abnur Tharn during her adventures in the Imperial City. Ayrenn has seen the rot at the heart of Tamriel and she thinks that it is the time for Elves to take the Throne in White Gold Tower in Cyrodiil. The tower was built by Elves long ago and Ayrenn believes that Elves must take it back to get rid of humans who can not rule Tamriel without blood and wars. It is time for Aldmeri Dominion to return peace and order into the region.
What’s great about this from a gameplay standpoint is that it gives each Alliance a distinctive feel and ambiance that each have their own methods and goals, and the player gets to experience this difference from inside each Alliance; they get to take up these methods and try to achieve these goals as they’re playing through the story of the Alliance War. It’s a hallmark of the Elder Scrolls series; there is no single truth about the world, that everybody sees things from their own vantage point. And it’s that legacy, of a rich and nuanced world, that we’ve taken up and expanded upon for Elder Scrolls Online.