The First War was a major galactic war that occurred roughly around 6000 BC, and raged on for nearly 25 years. Despite the fact that the war was mostly centered in the Orion Arm, the outcome of the war shaped galactic politics for thousands of years to come.
One outcome of note for the war, is the emergence of the Vystok Empire as a new super power after having originally been a low-tech backwater nation. After the seizure of Human, Coraean, Y'jarivan, and Exulian technology, the Chevaki were able to successfully reverse-engineer and improve their technologies.
The First War is widely considered to be the first major galactic war, surpassed only by the Second War in terms of severity.