Hydra System Massacres

The Hydra System Massacres were a conflict that took place during the Insurrection, sometime before 2518. Insurrectionist activity resulted in the massacres, but it is not known who massacred who or what the events were that led up to it.

In Halo: The Fall of Reach, Catherine Halsey refers to conflicts "from Mars to the Jovian Moons to the Hydra System Massacres and on to the hundred brushfire wars in the Outer Colonies", implying that the massacres predate said wars; but this seems unlikely, as the Insurrection only began in the 2490s.