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.
If you would have peace, prepare for war
This isn't intended to spoil anything for you -- there are no specifics on the war as it unfolds in Pandaria here. It's more about the nature of the war and how it flows from the history of the previous wars, which we've covered before.
To put it simply: Both sides believe themselves to be sinned against. The orcs of the Horde, who are the driving force behind its military, look back at their time in the internment camps as a great injustice, where they were stripped of their honor and treated like animals. Garrosh Hellscream's popularity comes from his implicit promise to return the orcs to greatness, to conquer this new world so that his people may live free on it, make use of the fruits of the land's bounty.
Those who are willing to go along with Garrosh's program come under the protection of the Horde. The trolls, goblins, forsaken, blood elves and to a lesser extent the tauren are lesser races in Garrosh's eyes, but they're wise enough to be part of the Horde and so will share in the spoils when the Horde conquers Azeroth entire. Everyone else is to be pushed aside. Each of these races has a reason to be part of the Horde, whether it be ties of respect forged by Thrall, a need for the protection and resources of the Horde war machine in order to lay their own plans, time to rebuild, or a sense of betrayal by former allies.
This common cause is fraying to some degree, as the leadership of each faction in the Horde realizes just how total the war Garrosh wishes to wage really is, but at present, it's still considered to be better to be on Garrosh's side than in his path. The Alliance hasn't successfully countered the Horde yet since the end of the war in Northrend, and even the most conflicted faction leader has to recognize that.
Sylvanas, while personally no fan of Garrosh and recognizing that he bears no lover for her Forsaken either, benefits from the Horde's aggressive new war footing and uses it to expand her own holding. If conflict ever comes between her and the Horde's warchief, she's extended the Forsaken's holdings down into Gilneas, across the entirety of Alterac and into Arathi as well as up into the Western Plaguelands. Her people's current strength is entirely due to Garrosh's program of total war.
Nothing new under the sun
The problem with this program is that it's nothing new to Azeroth. The Horde has waged total war across Azeroth twice before. The first time, it only succeeded after a regime change and the assassination of the enemy leader. The second time, the Horde war machine fell apart in an orgy of private agendas and self-destructive retribution, after having made so many compromises to honor for the sake of victory that there was no honor left.
The orcs of today remember the internment camps but have chosen to forget how they arrived in them. They remember the names of their great heroes -- Doomhammer, Grom Hellscream, Kilrogg Deadeye -- but they've chosen to forget the actual people and instead have embraced their legends.
Gone is the wariness of a Saurfang for the legendary recklessness of Hellscream. Gone is the awareness of the debt of honor owed Alexstrasza's brood for Doomhammer's enslavement and torture of the Dragonqueen and her consort via the Dragonmaw Nekros and Zuluhed the Whacked. The assumption by most orcs is that they are a new organization, a new Horde and thus no longer bound by the crimes of the Old Horde -- yet they make heroes of that Horde's leaders, name cities and towns after them, and idolize them in the very next second.
This is the mindset of the modern orcs, made up of children of the camps, who neither remember nor care how they arrived on Azeroth. Embittered by their treatment, they determine themselves unwilling to pay the price for their ancestor's crimes yet drape themselves in the glories of those same ancestor's actions. It would be one thing if they repudiated those ancestors -- sometimes their parents -- for their crimes, but they do not. They want all the glory they see in the tales of their parents' invasion of Azeroth but none of the dirty, onerous work of understanding what their ancestors did and the cost. They desire to repeat history but this time to be the winners, to conquer and crush their enemies, to carve a new peaceful land out of Azeroth by destroying those who stand in their way.
Standing on the brink
The Alliance, on the other hand, has real difficulties going into this war. The first is the resentment of the humans for the actions of those same ancestors. To be a human on Azeroth today is to be a survivor, and one cannot be a survivor without things to have survived.
Humans have survived the destruction of the ancient Seven Kingdoms, pared down to one remaining nation. Arathi and Alterac are utterly gone. Gilneas is a cursed land invaded by the Forsaken and the natives turned into worgen. Lordaeron is a Lich Kingdom parceled out between factions of undead, and Kul Tiras is lost in the open ocean. Dalaran has abandoned them to float in the clouds, a nation of magicians who care nothing for humanity or its fate.
Alone, Stormwind endures, the memory of the orcs invasion ever fresh in the mind of its king and its people. Anduin Wrynn may be wise, noble and forward-thinking, but his father's rage and hatred of the Horde is a lot closer to the memories of his people, who saw their city burned and who fled one step ahead of the green monsters that murdered and burned.
We look at the Third War and see squandered potential, a possible peace between the orcs and the humans, but most humans barely even know what happened. To them, it was a far-away battle on a distant land that most of them will never see. To them, the Third War was the destruction of Lordaeron, first of humanity's nations, the refuge they retreated to when Stormwind was destroyed. Telling them that the Forsaken are the rightful inhabitants of Lordaeron means nothing to them, even before the Forsaken expanded into nations that had never been part of Lordaeron. The Forsaken are dead, and dead people stay in the ground. It would be like telling the people in The Walking Dead that they should just let the zombies eat them.
To the humans of Stormwind, the Horde is a collection of monsters that eat people and burn their cities down, that invade your lands and kill your families. Putting the orcs in camps was foolishly soft-hearted. They should have been destroyed. The orc children who left those camps left embittered at humanity's insistence on punishing them for the First and Second Wars, and the human children of burned Stormwind and the ravaged Seven Kingdoms are embittered by the existence of orcs on their world years and years after the loss of Lothar and the defeat of Doomhammer at Blackrock Mountain.
Hanging together or hanging separately
This is exacerbated by the tendency of each race to go it alone up until now. This comes to a head most recently when we see the night elves attempting to stand alone against the entire Horde war machine, with trolls, tauren, even Forsaken fighting together with orcs to push the night elves out of Ashenvale after having already done so in Azshara (once entirely night elf territory and now fully in the hands of the Horde). The Horde was only balked in Stonetalon due to the humans of Theramore cutting a supply path across the ravaged Barrens and into Stonetalon. (Ironically enough, Theramore was one of the few bastions of humanity that has fought alongside orcs, since Theramore was founded by the forces Jaina led to Kalimdor and into battle atop Hyjal.)
That in turn created a whole new wave of Horde animosity toward the Alliance, in part because it involved members of the Horde who'd never had a personal reason to hate humans or the Alliance before. The attack on Camp Taurajo meant that for the first time, the tauren had seen their own dead in a manner reminiscent to the centaur attacks that pushed their people to the edge of extinction. Whatever their leadership may think of Garrosh Hellscream, Camp T became a rallying cry among the younger braves.
Fruitless to explain to them that across the world, many young humans felt the same way about the Horde, thanks to the destruction of Southshore. What did Southshore mean to them? They never saw it. But tauren dead on the ground in The Barrens, once their home -- that they saw and viscerally understood.
Reaping the whirlwind
What we see in Mists of Pandaria is the result of each faction's finally abandoning any pretense. The Alliance forces in the Jade Forest are unified. They are composed of members of every race. Worgen man the guns. Draenei snipers work alongside night elf tacticians and dwarf cannoneers. Gnome healers get human infantry patched up.
In a way, the Horde has succeeded in remaking the Alliance. Each side can list off its enemy's atrocities and crimes against them, and who started it pales into insignificance compared to that list of grievances. For every Camp T, internment camp memory, or Garithos on the Horde side, there's a Southshore, a Swamp of Sorrows, a burned Stormwind or an invaded Gilneas on the Alliance side, crying out for more blood because war is seen as the remedy and not the cause.
The younger members of the Horde have wanted an enemy they can recast the First and Second War with, and this time, recreate the glory of conquest without the dishonorable, blood drunk demonic elements they've chosen to pretend didn't exist. Now we're going to see that mundane hate is sufficient for all manner of atrocities and that decades of animosity cannot be wiped away by one or two desperate truces and temporary choices to work against a common foe. This is a war that waited to be fought until all the children were old enough to hold swords and axes but too young to know why not to do so.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.