The War of the Ancients is a war that took place some 10,000 years ago that shook the face of Azeroth forever. When Queen Azshara and her Highborne worked recklessly with the magics of their Well of Eternity - crafted by the titans long ago, the fount of life in the formerly perfect continent Kalimdor - it drew the attention of the Dark Titan, Sargeras, and he launched an invasion of Azeroth.
This war also happens in two separate realities, one which is the original reality between the night elves and the Highborne, the other between multiple other races. Korialstrasz, Ronin, and Broxigar travel back in time and participate in the war, and there is also the aid from multiple other races of Azeroth such as the tauren.
Sargeras corrupted and deceived Queen Azshara, and she allowed her high councilor Xavius to open a portal for the Burning Legion to come through to invade Azeroth. Slowly the portal would grow larger, soon to allow Sargeras access to Azeroth.
The War of the Ancients was the largest Burning Legion invasion of Azeroth until the Legion's third invasion, which was several times larger than the War of the Ancients.