History
The War-Moon orbited Tarnax IV, the Skrull throneworld. It had training grounds made to look like alien cities (e.g. Shi'ar, Earth, etc). The moon was a training facility for Skrull spies-in-training, dopplegangers who wore the forms of superheroes and villains that they were supposed to replace. Skrull operatives wielding the full array of those individuals' powers.[2]
The time displaced X-Men found themselves on this moon and ended up having to fight the trainees. Arriving right before Galactus, they soon found themselves having to race to get off the moon before the destruction of Tarnax IV. In the process they tried to save as many Skrulls as they could on the moon, while Professor Xavier tried to convince Galactus to spare Tarnax IV without success. Among the Skrulls, a story spread to some of them, about how the X-Men fought on the side of the Skrulls against Galactus.[3][4]