Out of the four main bending civilizations in Avatar: The Last Airbender, the Fire Nation distinguished itself as the most technologically advanced. Its steam-powered automobiles, weaponry and navy played a huge role in the expansion of its empire, as the other nations were almost helpless in the face of such innovative war machines. Yet being the most technologically advanced country didn’t mean the Fire Nation had a monopoly on engineering talent. In fact, its greatest weapon was delivered by none other than Sokka of the Water Tribe.
Apart from being a capable warrior, Sokka was also one of The Last Airbender’s smartest characters. Using the bare minimum of tools at his disposal, he imitated both earthbending and firebending, figured out how to disable the Fire Nation's giant drill and singlehandedly birthed the idea of an invasion on the day of Black Sun. Sokka may have been the series' main source of comic relief, but his intelligence was nearly unparalleled.
It's no surprise that Sokka instantly hit it off with the unnamed mechanist introduced in The Last Airbender Book 1, Chapter 17 "The Northern Air Temple". Before they met, Sokka had already shown admiration for his modifications to the Air Temple. The duo quickly bonded over their love of bad jokes and before long, the mechanist took Sokka under his wing, shared some of his prototypes and they continued to bounce around ideas for new inventions. Sokka was in his element -- but his fun couldn't last long.
The threat of an impending Fire Nation invasion forced the Air Temple inhabitants to prepare to defend themselves. Of course, Team Avatar joined in those preparations; as benders, Aang and Katara were able to join the fight directly, but Sokka put his mind to work figuring out how they could take advantage of their altitude. He ended up perfecting the mechanist's design for a war balloon and flew it with his mentor into battle.
Their war balloon quickly turned the tide in their favor but to secure a decisive victory, Sokka sacrificed the balloon's engine to trigger a massive explosion. His plan worked but without an engine, there was no way for the balloon to remain afloat. Sokka and the mechanist were rescued just in time by Aang as the balloon sank all the way to the foot of the mountain. Unbeknown to the jubilant victors, it was discovered by the Fire Nation.
The Fire Nation kept its latest asset under wraps until the day of Black Sun. After the eclipse was over, it unveiled a truly terrifying sight: Sokka's war balloon had been mass-produced and the Fire Nation now possessed an entire fleet of them. To make matters worse, some experienced technicians had capitalized off Sokka's design and built airships -- larger aircrafts that made up for their slow speed with crew-carrying capacity and increased durability.
The Fire Nation used these new war balloons exactly as Sokka did in his first and only balloon trip in The Last Airbender: air-dropping bombs directly on top of its enemies. Within a matter of seconds, the entire invasion fleet had been destroyed.