If the Fire Nation would've followed Sun Tzu's advice, they'd have been victorious a long, long time ago — before the return of the Avatar. Good thing they didn't.
Sun Tzu advised swift, efficient battles to make an effective war.
"There is no instance of a country having benefited from a prolonged warfare." & "Let your object be victory and not lengthy campaigns."
Also the Fire Nation's sovereign was not the one imbued with the Moral Law, although that country had the greater general, a stronger army, and more discipline. However, the Fire Nation and Team Avatar (as Sokka calls them) did have an equal advantage of having trained men, and greater constancy of reward and punishment. These indicators were how Sun Tzu could said he could predict victory or defeat.
So who is Sun Tzu? No one really knows. He is a legend like unto Robin Hood or King Arthur, his history written centuries after he lived, and he was most likely a member of a clan expert in arms and fighting. He is attributed with being a general and assigned training duties for his sovereign.
"Supreme excellence consists in breaking the enemy's resistance without fighting."
"If you know the enemy and know yourself you need not fear the result of a 100 battles."
During much of the first part of the series, Aang is trying to find himself, learning how to be the Avatar. He often comes into contact with the enemy through Prince Zuko or his sister, Azula. He also has traveled to the fire nation.
Sokka exemplifies Sun Tzu's advise on planning and stratagem. He uses his mini-vacation to go to the Library! so he can learn how to defeat the fire nation. He's usually making plans to rescue people, prepare the invasion and learns how to be a master swordsman.
He takes to heart the advice of Sun Tzu: "Ponder and deliberate before you make a move."
To be victorious, Sun Tzu says there are five essentials:
• He will win who knows when to fight and when not to fight. — Like Team Avatar did by planning the Invasion when the Fire Nation would be weakened; retreating when they realized victory wasn't at hand.
• He will win who knows how to handle both superior and inferior forces. — Like Team Avatar did by dealing with Prince Zuko & Azula as well as Jet and his gang.
• He will win whose army is animated by the same spirit throughout its ranks. — Like Team Avatar regained after Katara, who had expressed disapproval of Toph's scams, seeks to help her on one last one. Or when Katara & Sokka rekindled their siblingness after traveling through the great divide.
• He will win, who prepared himself, waits to take the enemy unprepared. — Like Team Avatar was forced to do after a failure, and regrouped to help Aang overcome a fear of firebending.
• He will win who has military capacity and is not interfered with by the sovereign. — Like Team Avatar overcame by restoring peace to the Earth kingdom capital, and by not having the parents of Team Avatar get in the way of their plans.
Learning all this throughout their experiences of air-, water-, earth- and fire-bending; Team Avatar knows the Art of War. Plus, having a flying bison on your side is always a plus.
" I'm just a guy with a boomerang, I didn't ask for any of this flying and magic!" — Sokka
Sokka could write his own treatise on war and fighting.
"In order to take out the Fire Lord- or in this case, the Melon Lord- our timing has to be perfect. First, Suki and I will draw his fire. Then, Katara and Zuko charge in with some liquidy-hot offense. And while the Melon Lord is distracted, Aang swoops in and... BAM! He delivers the final blow."
Sokka could write his own treatise on war and fighting.
"In order to take out the Fire Lord- or in this case, the Melon Lord- our timing has to be perfect. First, Suki and I will draw his fire. Then, Katara and Zuko charge in with some liquidy-hot offense. And while the Melon Lord is distracted, Aang swoops in and... BAM! He delivers the final blow."

