The Burning Forest Plan (燃える森計画, Moerumori Keikaku) was a project created by Mōretsuna Uzumaki as a way to take revenge on Edagakure for destroying Uzushiogakure. The plan took place over several decades in order for Mōretsuna to gain the necessary reputation, trust and power required for it to work.
The Plan in Theory
Several decades before the start of Sanger Hiden, Mōretsuna was a loyal shinobi to his village who was skilled in espionage and assassinations. When Eda forces invaded the Uzushio, Mōretsuna became trapped in a collapsed building near the beginning of the invasion, preventing him from assisting his village. Following his rescue two days later, he saw the village in ruins. Over the next week, he saw that not only was Uzushio destroyed, but so was the entire Land of Whirlpools. This led to Mōretsuna having a mental breakdown and swear vengeance upon Eda.
Taking the name Mukui, began to study the Land of Rainforests and much to his surprise, learned of a giant, golem-like creature sealed somewhere within the country's rainforests. Knowing he would not be able to find the golem's sealing place easily, Mōretsuna managed to infiltrate Eda under the guise of one of their shinobi.
Now that he was safely integrated with Eda, Mōretsuna could begin his plan in earnest: he would begin by researching the hidden location of the golem and when presented with an opportunity, seal it within himself. He would then take the time to master the power that the golem granted him, while waiting for an opportunity for the village defenses to weaken such as during a war. While Eda was in a weakened state, he would unleash the golem in the middle of the village. As the village forces were distracted, he would systematically slaughter them which would simultaneously make it harder to stop the golem.
The Plan in Actuality
In order to learn the location of the golem's sealing, Mōretsuna spent several years gaining the trust of the village higher ups, until he eventually became one himself and learned of a hidden shrine deep in the unexplored parts of the Land of Rainforests. Around ten years before the start of Sanger Hiden, Mōretsuna sent a disguised clone in order to unseal the golem. Upon its successful unsealing, the golem began to rampage and started to make its way to Shishō.
Because he was highly trusted, Mōretsuna was put in charge of team that was to reseal the golem. Upon driving the golem back to the heart of the rainforests, he sealed it within himself using his clan's special techniques much to the surprise of the rest of the sealing team. Upon his return, Mōretsuna was regarded as a national hero and was invited by the Rainforest Daimyō to be his bodyguard.
After manipulating the daimyō for several years in order to cause a war, Mōretsuna decided that he needed to be in direct control after his efforts constantly failed. Eventually, he secretly caused a national emergency by killing the daimyō, his heirs and several potential successors. In order to cast suspicion away from himself, Mōretsuna non fatally poisoned himself and several others, while subtly casting the blame on the Land of Water. Some time later, Mōretsuna was made the temporary leader, of the Land of Rainforests and eventually granted full daimyō status after how well he performed.
Upon the advent of the Fourth Shinobi World War, Mōretsuna attempted to convince Kegawa, the most recent village head of Eda to assist the five great shinobi countries only to fail. Bidding his time again, he convinced Kegawa to create a terrorist organization under the pretext of weakening the military might of other countries. A few months after the end of the Fourth Shinobi World War, Shizen'na was up and running and ready to begin its missions.