The War is a major global conflict which is occuring in the Avatar universe. Going on for a century, the War was started by Fire Lord Sozin, who wished himself to rule the entire world. The War is actually only supported by a very small amount of the population; virtually only Fire Nation elites actually wish to fight in it.
Prelude
The first Earth Kingdome territory taken by Sozin despite Avatar Roku's warning.
One hundred and twelve years ago, Fire Lord Sozin started having delusions of grandeur to be Fire Lord not only of the Fire Nation, but of the entire world. However, he knew that he could not act upon this as long as Avatar Roku was alive, as he would stop him. That year, Roku died, and the new Avatar was born into the Air Nomads as Aang. Sozin, however, delayed his plans to await the coming of a comet that would bring his Firebender armies near-unlimited power.
First Strike
The destruction of the Air Temple by Fire Lord Sozin.
One hundred years ago, Lord Sozin decided to launch his attack due to the arrival of Sozin's Comet. With the power of the Comet, Sozin's Firebender armies launched a genocide on the pacifistic Air Nomads in an attempt to kill the new Avatar, Aang, and to halt the Avatar cycle. However, Aang had run away the night before, thus escaping the carnage. All other Air Nomads were believed to have been killed in the attack, leaving Aang the only known Airbender left alive.
Early Stages
The results of the war in the Earth Kingdome.
Not much is known of the War's early stages, but it is known that the Firebender armies advanced from the West and headed East. It is probably during this time that Taku was attacked and destroyed. The Fire Nation continued to advance East. The Firebenders could not attack the Northern Water Tribe because of its dams and locks. Around this time, Sozin died and was replaced by his son Azulon.
The War Continues
Azulon continued the imperialist war as the Firebenders pushed farther and farther East and South. In the South, the Fire Nation attempted to conquer the Southern Water Tribe with a single ship but failed - a single village fought the ship off and wrecked it. Meanwhile, General Iroh pushed his armies to the very walls of Ba Sing Se itself, launching a massive siege on its walls.
Azulon, too, began to grow old, though his temper remained. When Iroh abandoned the Siege of Ba Sing Se because of the death of his son, Lu Ten, second-born Ozai demanded that Azulon give the birthright to him. Azulon threatened to kill Zuko in retaliation, according to Azula, but under "mysterious circumstances", Azulon died the next night. Ozai's wife also disappeared. His dying wish was that Ozai succeed him. Thus, Ozai was crowned Lord of the Fire Nation.
Recent Years
In recent years, the war let up. Iroh's forces retreated from Ba Sing Se's area, although they remained in control of half of Full Moon Bay where they began construction of the Drill. During Ozai's reign, the Army retreated, easing up control on certain areas of the Earth Kingdom. The Northern mountains were let alone, allowing pilgrims to move in. The Western shore of the Earth Kingdom was put under tighter control and Prince Zuko began his search for the Avatar. Zhao, an eager military man, also pursued such goals while plotting another conquest - the Northern Water Tribe.
As the Avatar traveled north to the Pole in attempts to learn Waterbending, Zhao was promoted to Admiral and ordered the entire Navy to advance on the North Pole. So they did, getting only a few days after the Avatar arrived. Meanwhile, a small militia attacked the Northern mountains, only to be beaten back. However, the Siege of the North was unsuccessful due to the Avatar's intervention, and the fleet was destroyed. Zhao was killed by La. With the Fire Nation's main ambitionist out of the picture, Ozai continued the slow and steady advances on all fronts, while discreetly sending his daughter to search for the traitorous Iroh and Zuko.
The Fire Nation's next major battle would be at the walls of Ba Sing Se once more. The Drill, a powerful war machine, attacked the wall (unfortunetely, just as the Avatar was present). Though the machine went through the Wall, it was destroyed by the Avatar, and the attack failed. Fire Nation forces retreated from Full Moon Bay and the Ba Sing Se area while a small team of three infiltrated the capital.
The three were the Princess Azula, Mai, and Ty Lee, a trio of the Fire Nation Academy's best. Clad in Kyoshi Warrior uniform, Azula infiltrated the court of the Earth King and undermined the Dai Li. She eventually convinced Long Feng to launch a coup d'etat against the Earth King. In the end, she double-crossed him and assumed control over the capital. With Ba Sing Se under the Fire Nation's control, only the Water Tribes and a small area of the Earth Kingdom were left to be conquered by the Firebenders.