Cosmic Era (C.E.) 70, the "Bloody Valentine" tragedy escalated into a war between the Earth Alliance and ZAFT. It became severe. Making many sacrifices, they achieved a truce after the second battle at Jachin Due. However, this truce didn't subdue the conflicts between Naturals and Coordinators completely.
During the war, Shinn Asuka was involved in Earth Alliance's intrusion into Orb. While he was escaping from the fire, he lost his parents and little sister before his very eyes. When he was deep in sorrow gripping his sister's cellular phone, which is his only memento of her, a Gundam, which was the principal cause of the war, was flying over his head. Being disappointed, he left Orb and went to PLANT.
In C.E. 73., he became a ZAFT soldier.

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Prequel: Kidou Senshi Gundam SEED
Sequel: Kidou Senshi Gundam SEED Destiny Final Plus: Erabareta Mirai
Side story: Kidou Senshi Gundam SEED C.E. 73 Stargazer
Summary: Kidou Senshi Gundam SEED Destiny: Kudakareta Sekai
Summary: Kidou Senshi Gundam SEED Destiny: Sorezore no Tsurugi
Summary: Kidou Senshi Gundam SEED Destiny: Sadame no Gouka
Summary: Kidou Senshi Gundam SEED Destiny: Jiyuu no Daishou
Sequel: Kidou Senshi Gundam SEED Freedom