It is unknown how the war started, though how it ended is known. During the war, the Dragon Army was led by a great Seadragonus Giganticus Maximus named Merciless. It is unknown who led the Vikings, though it can be inferred that it was the Chieftains of the tribes that led them. During the majority of the war, the human race was losing, very badly.
List of Events
One day during the war, Merciless using his Seadragonous Giganticus Maximus Powers, foretold the future that he would be killed by a boy named Hiccup. Merciless learned that a boy named Hiccup (Hiccup Horrendous Haddock I) lived on a island in the Archipelago. Whether this island is Berk or not is unclear. Because of this, Merciless sent a fellow Sea-Dragon, the Wodensfang, to kill the Hiccup boy.
Going to Kill Hiccup & The Rescue
When the Wodensfang flew over to the island to kill Hiccup the First, he flew rather low to the ground and got caught in some branches. The Wodensfang was caught in those branches for two days, with a bad wound in his chest and growing weaker. He was eventually rescued by Hiccup the First, of who he was surprised to find was just a small boy, eight or nine years of age. The Wodensfang was surprised that a small human boy would rescue a dragon, due to the fact that humans and dragons were mortal enemies in that time. Hiccup the First nursed Wodensfang back to health, and he flew away to the Dragon Army again.
Getting to Know Hiccup
Unbeknownst to anyone, the Wodensfang returned to the island Hiccup the First lived on. Wodensfang taught Hiccup the First the language of the dragons, Dragonese. In return, Hiccup the First taught the Wodensfang Norse. Hiccup the First also became the first human to ride on a dragon's back when he rode on the Wodensfang. The more the Wodensfang and Hiccup the First became closer and closer, he saw things that was special within the human race that dragons lacked. These included: love, imagination, creativity, and communication through language. Wodensfang realized that dragonkind could learn much from humanity. Unfortunately, the Dragon Army was annihilating humankind to extinction, so the Wodensfang decided to save the human race.
Stealing the Dragonsword
The Wodensfang realized that in order to save humanity, he would have to steal the Dragon Jewel, which was hidden inside the pommel of the Dragonsword which was hidden in a firepit that was located underneath the mountain that would eventually have Flashburn's School of Swordfighting built on top of it. The firepit was also guarded by a dragon the Wodensfang described as "a far more terrible dragon than I am." Wodensfang stole the jewel and the sword; then gave them to Hiccup the First. Hiccup the First promised Wodensfang that he would not use the jewel for evil and that he would make sure that humans and dragons lived together in harmony.
Ending the War
Merciless then flew to the island where Hiccup the First lived to personally assassinate him. When Merciless got there, Hiccup the First held up the jewel to Merciless and forced the great Sea-Dragon to call off the Red-Rage and leave the Barbaric Archipelago forever to live a life as a loner.
Merciless made good on his promise to never return for around 1,000 years. He returned to the Barbaric Archipelago in How to Train Your Dragon as the Green Death, where he was ironically killed by a boy named Hiccup.
In return for the Dragon Jewel and its secret, Hiccup the First put a mark (the first Dragonmark) upon his forehead and once again swore on that mark he would never use the jewel's power for evil. Hiccup the First then established the Kingdom of the Wilderwest where he became the first King. Hiccup the First used the Dragon Jewel to train dragons, who were trained with respect and dignity. The Kingdom of the Wilderwest was a place where humans and dragons lived in equality and harmony. Though over the millennium, each king treated dragons worse than than the last. This eventually culminated in Grimbeard the Ghastly, who rejected the Dragonmark as a boy and enslaved both humans and dragons to build a magnificent city on the Island of Tomorrow. This caused the Wodensfang to believe that he was wrong to trust humanity with the secret of the Dragon Jewel. These cataclysms planted the seeds for the Dragon Rebellion.