That's it, this war ends now, once and for all!
Phoenix King Axrendale returns from his secret journey; and reveals his secret: The Sun Warriors have joined his cause, and over the last few years have undertaken an extensive dragon breeding project. The Dragons have been trained for battle and now follow Axrendale. He gathers together all of his strongest armies; The Imperial Firebending Guard, The Fire and Combustion Sages, The Yu Yan Archers, the Dai Lee Agents, and forms them into a massive army that is then transferred to a massive War Balloon Fleet, escorted by the Sun Warriors, riding their dragons.
Auxillary forces from the Fire Nation's allies encamped on the earth Kingdom's beaches lauch a feint attack in the South of the Earth Kingdom. As the various armies of the Earth Kingdom and Swamp Tribe head to repel them, the Air Armada, commanded personally by Phoenix King Axrendale, bulls it's way through the North West of the continent and makes it's way steadily towards Ba Sing Se.
It bombards anything and everything that it comes across, and leaves a trail of devastation in it's wake.
Meanwhile, occupation based forces under the command of Princess Azula move steadily behind the Armada, moving in to occupy vast swathes of the Northern Earth Kingdom.
Unexpectedly, the Armada bypasses Ba Sing Se, and heads towards the soldiers camped along the East of the Earth Kingdom. Meanwhile the Water Tribe's new navy launches an unexpected attack against said soldiers. It is driven off, but proves to have simply been a distraction - when the Armada hits the soldiers from behind, they are helpless.
Most of the Northern Earth Kingdom is now under Fire nation control, and the south is now heavily contested.