Sombra declared war on Equestria after he regained control of the Crystal Empire. In an attempt to stop the war, Celestia visited Sombra to negotiate a peace treaty, but Sombra murdered Celestia at the meeting. Luna taking control over Equestria, retorted to the murder of her sister with all out war. The two had advanced tech before the war, the first mechanized tank being introduced to the public 13 years before the war. The Main Six separated to help fight in the war, not being allowed to talk to each other.

Shout out to Rogue Brony on google+ for the idea for this story, check out his comic series, also titled "The Mechanized War".