If there's a force difficult to quantify as a faction, or organization, it's 'The Siege'. Led by Tiamat, even the very name 'The Siege' was coined by a human reporter commenting on how the entire human race had been laid siege by the dragon. The description stuck, and humanity came to call the war against Tiamat a war against 'The Siege'.
But Tiamat represented a force like no other the world had ever faced. The dragon was very much an army of one, and it made this perfectly clear in it's declaration of war. It's first act; to kill every single man, woman, and child in north, central, and south America in a hail of soul seeking homing beams. The dragon would go on to personally erase Great Britain from the globe entirely, before finally being confronted by The Divine Golems high above the ancient underwater civilization of OZ. This mighty clash led to over 90% of the world's life vanishing.
Tiamat was forced to withdraw from The Earth, unfortunately for the remnants of humanity, the dragon had discovered a derelict trans-dimensional weapons system known as 'Arsenal' in the short time the dragon drifted between life and death, before making it's evolution into Tiamat. Arsenal was designed by an ancient alien civiliazation to oppose the aliens who masterminded Tiamat's very creation, so ironic it is then, that it would ultimately fall into the dragon's ranks.
Following the fight against The Divine Golems, Tiamat had Arsenal deploy it's Girtarion kill bots to The Earth's surface to hunt down the remnants of humanity. Mankind would have gone extinct then and there, if not for a treaty Tiamat had made with the child like people of Luna. The Lunarians lived on the moon in perfect peace for centuries prior to the coming of Tiamat, and had developed a level of technological advancement far above what the people of Earth had. Not wanting the Lunarians to potentially intervene in it's quest, Tiamat made a treaty with them. In exchange for Luna's neutrality, Tiamat provided them with Phoenix, an immortal flame that would serve as Luna's ambassador to 'The Siege'.
Phoenix took pity upon the remnants of mankind, and declared Bastion off limits to Arsenal's Girtarion kill bots. Phoenix also created Dreya, to watch over the people of Bastion. As for Tiamat, the dragon converted the last of The Global Security Network's orbital staions into it's den. It renamed the station 'Avalon' and outfitted it with a Lunarian built cloaking device. The few humans who remained on the station were altered by the dragon into 'dragonkin', humanoid reptillian creatures loyal to their creator. Here, Tiamat has remained for 250,000 years, allowing it's wounds, and the wounds of the Earth to heal. Now that the Earth's vitality has been restored, The Return of the dragon is at hand.