People fear change. They fear abandoning their way of life, even if it's one where they steal land and kill innocents -- perhaps especially, because that's what they do. People who feel that they have been the victim are often motivated to ensure that doesn't happen again. They take actions to prevent it and in so doing become the monsters they were fighting.
There's an NPC in the Mists of Pandaria beta who is a perfect microcosm of this. When asked why she is so eager to attack the Horde forces in Pandaria, she replies that she grew up in Southshore and that her parents (specifically, she says "what's left of them") are now buried there.
It is that struggle to avenge the losses of the past that drives the evolving war. Those of you who played the Swamp of Sorrow in Cataclysm will remember the Horde/Alliance struggle there, which ends in no real change and no development -- each side gets to burn the other, and there's no definitive change to the status quo. This is despite the introduction of people like Joanna Blueheart, whose statement "I'm not here to settle a grudge ... I'm here to win a war" seems hollow when she gets to do neither.
Over the past years of our playing World of Warcraft, the tenuous peace from the end of the Burning Legion's invasion of Azeroth atop Mount Hyjal has unraveled. Countless skirmishes in Alterac Valley, Arathi Basin, Warsong Gulch, the Eye of the Storm in Outland, and at both the Strand of the Ancients and the Isle of Conquest in Northrend expanded to include the invasion of Gilneas, bogged down in endless skirmishes, and the struggle over the Twin Peaks in the Twilight Highlands. Battles waged across Wintergrasp's Titan ruins and to decide who controls the prison of Tol Barad, meaningless taken by themselves, paint an evolving picture of war. Now, we bring that war into Pandaria.