It really isn’t something I expect would happen either, unless there is a large chain of assaults by multiple other powers, which would probably require co-operation and co-ordination, something rare between the great powers. I mean this completely as a “What if…” scenario.
Looking at the map, one plausible (albeit unlikely) scenario I see which could collapse the Alliance would go something along the lines of:
- Major Swarm invasion of northern Alliance territory. Many Alliance fleets mobilized to defend against Swarm.
- Order forces exploit less defended borders to attack Alliance territory. Alliance forces in main region now focused along the two fronts. Secondary regions also under invasion.
- Remaining uninvaded Alliance borders are bordered by the Federation. The federation may then use weakened Alliance borders to expand their borders and/or attack major targets, further weakening Alliance defensive
- Secondary regions of the Alliance, weakened by previously mentioned powers, have come under heavy assault by two major enemies of the Alliance: the Tribe and the Rebels.
By this point every region controlled by the Alliance would be under attack by at least 2 Great powers, with an opening for a third to attack. Nomad and Parasite forces are generally too far away for front line invasion and also likely maintain a neutral relation with Alliance. The Empire has a positive relation with the Alliance and would likely not attack them, military support seems unlikely though.