The War Against the Nightmare was a conflict fought against the Emerald Nightmare started by Xavius as he attacked Azeroth soon after the end of the War against the Lich King. People could not wake up, and were lost in nightmares, without any way to let them out. A strange mist spread through the lands, a herald of the Nightmare. Shadowy figures started appearing throughout the world, and eventually even attacking the people. Later, the sleepers themselves turned into sleepwalkers, attacking everybody nearby as they relived their worst nightmares.
Meanwhile, the Nightmare Lord had turned a great part of the Emerald Dream into the Nightmare, and though nature's defenders did not stop in their attacks, they seemed to have no effect in stopping the corruption, while some became corrupted themselves. Malfurion Stormrage himself was trapped in the Nightmare, though with the help of some of his friends he was able to escape. Contacting every sentient and wild being that eluded the forceful slumber and remained awake in the physical world, Malfurion asked the survivors to fall asleep and lend their dreamforms to attack the Emerald Dream. The dreamform of Varian Wrynn led the troops.
In the Emerald Dream, the dreamform army battled against the Emerald Nightmare's army of corrupted Nightmare creatures. The dreamform army pressed on even when the Emerald Nightmare used a psychological tactic that had the enemy taking the forms of fallen comrades and loved ones of Azeroth's defenders. Yet things took a turn for the worse when the Emerald Nightmare started sending sleepwalkers to kill the dreamform armies' corporeal forms in the waking wold, causing their deaths in the Emerald Dream. The shrinking dreamform army could only hold on until Malfurion and his comrades were able to defeat the Nightmare Lord directly.
On the waking world, and with the help of several druids, Malfurion was eventually able to destroy the Nightmare Lord's power source, both on the Emerald Dream and in Azeroth. With him gone, the nature and dreamform armies were able to rout and defeat the Nightmare, though a small pocket of it remained unconquered.