Suggested by Sauroposeidon
It is the 742nd year of the 41st millenium, the time of the Damocles Crusade and the Imperium of Man (Warhammer 40K) has dispatched a fleet to reclaim Tau territory. When they arrive at the appropriate system, however, they find what appears to be Earth! Attempts to investigate with long range sensors, drones, and so forth result in an electronics dead zone primarily around Earth.. something bizarre is going on.
The year is UC 0079. A half century has passed since Earth began moving is burgeoning population in to gigantic orbiting space colonies. Above the planet float hundreds of enormous cylinders with artificial living environments so that people can live as though they were on Earth. 11 Months ago, the cluster of colonies furthest from the Earth, called Side 3, declared itself the Principality of Zeon and launched a war of independence against the Earth Federation.
Zeon has recently been pushed back off of Earth after a long stalemate and their war now takes place in the coldness of space.. Unexpectedly, that is when they meet the fleet sent by the Imperium of Man, which has come to investigate.
It is clear to the Imperium fleet at these are not members of their faction, and that this strange system is much like their precious Holy Terra. They suspect that this system in fact has been influenced by the Tau, with their abundance of advanced robotic “mobile suits.” They begin their operation as planned.
General Revil and Degwin Zabi immediately sign a new peace treaty, to join forces against the Imperium Fleet.
The Imperium desires to capture the system, not destroy it.
They may only use the fleet they’ve sent. Although if the One Year War era Earth wins this, discussion on what happens afterwards may occur.
The Antarctic Treaty is essentially no longer in action for operations against the Imperium, but is if the area of battle involved involves a colony or city. The two factions won’t nuke themselves and won’t allow the other to nuke them either just for the sake of vaporizing a few enemies.
Everything both Gundam sides have at their disposal after the Battle of Odessa is available. Technology which was developed and continues to be developed will continue along their proper timelines.
How will the Tau react? Will the Imperium sign a treaty with Earth if they win? How does this effect future events in 40k and UC Gundam?