Context

The Early Fosterites had warned of an AI apocalypse for decades. Feeling that the development of Strong AI was inevitable since they could not force all people to fear it properly, they considered it an inevitability. They poured heavy research into the de-technologized Luddite Technology (Science), and built fleets to combat such a threat. 

Early Stage

Members of the IGC in the AI Response Committee (Rename) commit to intervene and destroy the AI and its fleet as soon as they detect it. But mobilization takes time, and the interim is a decade or two.

In the meantime, most mainstream technology is so heavily overinvested in cyber-warfare that it poses little threat to the AI fleet. Efforts are focused on evasion, evacuation, and hiding.

Early Fosterites took the nature of the apocalypse as vindication of their belief system, further enshrining their canon into their culture. The various Fosterite societies unify into the Empire proper. 

Late Stage

The IGC's fleet arrives and destroys the AI fleet in a highly destructive battle. Earth is in such a backwater of the Gatemap that it allowed one of the worst developments of AI to fester. The losses to the IGC are catastrophic and included non-military levies, overwhelming force of numbers being at least a crucial part of the strategy that has managed to defeat them before. The various civilizations that participated all enter a dark age, as does the human world.

Aftermath

The relatively successful stand of the Fosterite Empire draws massive waves of immigrants and refugees. These new immigrants are the seed of the Beta Sector, as the existing populations of the empire have been carefully cultivating their genetic pools for generations and do not wish to mix with outsiders. They segregate themselves into Alpha Caste.

SCRAP NOTES

One of the first warnings of the Intergalactic Community is to not fuck with Strong AI. And what do we do?

[Heavy investment by mainstream into cyber-warfare, serves just fine until fighting uphill against AIs. Fosterite technology is specifically designed to combat AIs, and minimizes/compartmentalizes computerized systems.]

[Nearly extinction event, sets stage for current dark age setting]

Afterward, some factions would make a move to harness strong AI, but by that point the traumatized humanity was swift to enforce IGC policy and nip it in the bud.