The Steel Rebellion was the downfall of humanity's greatness after what they considered to be their strongest weapon and ally turned their back.
To explain it simply, the Steel Rebellion was the age where all of humanity's artificial intelligence malfunctioned and attacked their masters.
At that time, Artificial Intelligence were used in every technology imaginable an had helped humanity from day to day activities or fighting an all-out-war.
For decades humanity had depended on A.I. They never had considered a time in the future where they would stop using it. The created intelligence were so advanced and adaptive that an entire war cost humanity zero lives because their personnels were only comprised of robots, mechas, and spaceships fully controlled by A.I.
Without the need of food, morals, ethicals, and psychological interferences, the generals and the warmasters were able to create a grand strategy unlike ever before.