Combatants: Tri-Partite Pact and the Reapers
The arrival of genocidal Reapers saw the inhabitants of the cornered cluster realize there was only one way to survive, and that was to ally themselves against the common enemy. The Tri-Partite Pact was signed between the states of Joseo, Yuke, Velka (and the independent worlds) to fight Reapers.
Pact fleets spent most of the war on the run, hiding in the nebulae, conducting hit-and-run and rescue operations to retrieve civilians. The largest battle of the Pact was the Battle of Geert, where massed Pact fleets were discovered by Reaper scouts. Reaper capital ships forced the Pact fleets into battle, and broke their backs. Military remnants were sent further into hiding in the nebulae, while Reaper forces continued harvesting the civilians in peace.
As abruptly as it had began, the war ended on a quiet note. The fleets did not return until weeks after Reaper ships departed the cluster, and they found desolation and suffering left in the conquerors' wake. The period of time between 2186 and 2187 was called the Year of Isolation, as the cluster rebuilt and sought to regain connection with the galaxy at large through Future Corridor.