Welcome back! This is a sequel! I would heavily advise you to read the previous story, the Avatar of the Winter Moon, before this one so you can understand what's going on. But basically, Yue's the Avatar, the war didn't start with Sozin so Aang lived peacefully in his own time, Ozai did start it by trying to kill Yue on the night she found out she was the Avatar and then Yue worked her way down to the South Pole, much more powerful in this timeline, to find a master, defeating a Fire Nation attack in a great naval battle in the process.

The Caldera, 150 years Before Avatar Yue

Firelord Sozin looked out to sea, and gave a contented sigh.

"It's a wonderful day for a wedding," he declared.

"It is indeed," his friend said, stepping out onto the balcony. Avatar Roku looked around, wondering what was going on. "I always knew there was something between you."

"At least someone did, I didn't know Ta Min knew my name," Roku said.

"Trust me, she did."

Sozin had known- and used- this fact for a long time. He was interested in his friend's romantic happiness, of course, but that kind of information was also useful.

His thoughts must have shown on his face, because the smile dropped off Roku's.

"You don't look like you've brought me here to talk about the weather though," he observed.

"No," Sozin agreed. "I must confess that I hadn't. I have... a proposition for you."

Roku arched one eyebrow.

"Go on..." he prompted.

Sozin hesitated for a moment, unsure of how to broach the subject. Then he shrugged.

"I have an idea. Of how to make the world a better place."

"Another one?" Roku teased.

"Another one. My most ambitious yet. Look around you, Roku. What do you see?"

"I see a balcony. And a sunset."

"You see prosperity. Prosperity unequalled across the world, or in all of history. My people are the richest and happiest of all time."

"That's quite a big claim," Roku observed.

"I do not make big claims without good reason," Sozin pointed out.

Then he continued.

"But look across the sea," he said. "The Earth King struggles to maintain his authority far beyond the walls of Ba Sing Se. The Water Tribes freeze in their huts, they're still the same subsistence fishermen they've been forever. And the Air Nomads..." he trailed off.

"Well," Roku said after a pause. "I'm sure they'll welcome your criticism. So you're interested in the plight of the poor in other nations, is that it?"

"Something like that," Sozin agreed. "And of how their leaders squabble, and fight each other, rather than uplift their people. Just recently there was that Tsunami, wasn't there? And what did the Chief of fhe Water Tribe and the Earth King do to help their citizens in their hour of greatest need? They accused each other of doing it on purpose and nearly started a stupid war."

He turned to face Roku.

"So my plan is very simple. Unification."

"What?"

"If all the peoples of the world are united under the same government," Sozin pointed out, "then there will be no more war. And I can use my nation's industry and wealth, fuelled by the resources of their nations, to lift the world up and push it into a future that works for everyone."

"Right," Roku looked at him like he'd grown a second head. "And what happens if the other leaders refuse?"

"We make them see the light."

"A war, then."

"A war to end all wars. But hopefully it won't even come to that."

"It will come to that!" Roku exploded. "It absolutely will! I know the people of the other nations, I've met their leaders! They are proud people, and will not meekly accept their subjugation."

"Subjugation?" Sozin spluttered. "It's not-"

"It is! It will be seen like that, absolutely!" Roku told him. "You will be resisted. Bitterly, resolutely, resisted. And those people will be fighting in their own elements. There's a very real risk your armies and fleets will lose. And then where will you be? You'll be hated, denounced as a conqueror and a tyrant. And even if you do win, will you be able to keep it?"

He jabbed a finger at Sozin.

"You've said it yourself, the Earth King struggles to assert control over his Kingdom. But what makes you think you'll do any better? The Earth King is at least regarded as the rightful ruler of his people. You would be seen as a foreign conqueror. A tyrant, like Chin the Conqueror."

Sozin frowned at the thinly veiled threat. It was well known that the only reason Chin had failed was the intervention of Roku's predecessor, Kyoshi.

All ambiguity was removed a second later.

"And finally, if you do this, I would have to resist you too."

Sozin glowered at him.

150 years later, on the exact same spot, Firelord Ozai read a letter with increasing rage and disbelief.

"Morons," he growled.

The messenger who had delivered him the letter noticeably shivered with fear.

"I'm surrounded by morons!" Ozai repeated for emphasis. He rounded on the messenger.

"Can you believe it?" He asked sarcastically. "I launch an attack to kill the Avatar on the first night of the war. My incompetent generals fail to do that properly. Then one of them has her in the Earth Kingdom. The idiot lets her escape. Then she gets to the South Pole, and now I read that not only has one of my top admirals defected to the enemy, but the other has led his fleet to catastrophe!"

He was getting louder throughout this rant, causing the messenger to shrink in his boots.

"And not only that, he somehow managed to deliver those Southern Snow Savages their Chief back! And it turns out all our dragons are refusing to fight for good measure. I should make you an admiral, at least someone here can do their job properly."

"Y-yes, Firelord!"

"Fools." Ozai looked out to sea, his anger cooling enough for rationality to begin to seep back in. He knew he had been slightly unfair to his commanders. Most of them had performed well enough, and his armies were getting very close to Ba Sing Se itself by now. Clearly, the problem was the Avatar. She had been at the centre of every setback his forces had suffered thus far, so removing her was a priority. The failures of his commanders to get her throughout the initial stages of the war had removed the possibility of doing so while she was still weak and untrained. He was sure that she would even now have found a master and be practicing waterbending.

But he needed someone he could trust to do the job, and he needed them to be properly motivated.

Then he had an idea.

A competition. And he knew the perfect prize.

Turning back to the messenger, he barked orders.

"Bring me the Princess Azula, and Prince Lu Ten," he said. "They can both be of great service to me."