The Destruction of Project P.E.G.A.S.U.S. was the the battle that started the War for Earth. The outcome of the battle was the complete destruction of almost all the work done in Project P.E.G.A.S.U.S. and Hawkeye, Erik Selvig, and other S.H.I.E.L.D. agents getting their minds controlled by Loki.
Background
After Loki was defeated by Thor and fell into a portal, transporting him to Thanos' world, who tought him how to partialy control people's minds, without even being in the same wolrd they where in. He used this method on Erik Selvig and used it to discover certain things they where doing in S.H.I.E.L.D. to help him when he travels to Earth to rule it.
Attack
- "Dr. Selvig read an energy surge from the Tesseract four hours ago."
"NASA didn't authorize Selvig to go to test phase."
"He wasn't testing it, he wasn't even in the room. Spontaneous advancement." - ―Phil Coulson and Nick Fury[src]
Accepting the deal without fear of failure, Loki turned his focus back to Earth in search of a suitable agent, someone close to the Tesseract. He found Erik Selvig, a friend of Thor's and a renowned astrophysicist, who had been summoned to the task of unlocking the secrets of the mysterious cube. In time, Selvig came to understand some of the Tesseract's capabilities, and through Loki's influence he developed equipment that would focus some of that power. When the time was right, Loki activated the Tesseract, forming a temporary portal that pulled him through to its location on Earth. Despite Fury asking him to lower the scepter, he unleshed his powers at him and S.H.I.E.L.D. agents immediately open fire at him only to learn that bullets bounced off of his body. Nick Fury attempted to keep him away from the Tesseract after, but Loki managed to use the scepter to subjugate and control Hawkeye and Selvig, as well as a number of other agents by placing the tip of the scepter at their chest near their heart and allowing the power of the gem to overtake them.
Fury tried to flee with the Tesseract, but then decided to stay and allow himself to die with the cube if it meant stopping Loki as well. Selvig and Hawkeye informed Loki of Fury's intent, prompting Loki to gather the turned agents and leave with the Tesseract after attempting to kill Fury, avoiding Maria Hill's attempt to stop them. The energy it had unleashed consumed the area and the entire S.H.I.E.L.D. compound with it.
Aftermath
Now that Loki had started a war, Fury found this to be the perfect oppertunity to intiate the Avengers Initiative. One by one, Bruce Banner, Tony Stark, and Steve Rogers came together. When Thor managed to return to Earth, he joined as well.