In total, each side of the war lost a billion soldiers and citizens together.
The war was proceeded by the Second Amatria-Artria War in 10010.
The Amatrian Galactic Spherium sent intergalactic exploration probes to the Artria to expand their reach but a few months after they arrived in 9982, they got destroyed by the government controlling the Artria galaxy, the Artrian Galactic Republic. The Republic launched an entire squadron to the probes to eliminate them and succeeded. The Amatrian Galactic Spherium soon noticed no more signals coming from the probes and thought that they entered hostile territory in the galaxy and became destroyed.
The Spherium immediately recieved a signal from the Republic, with intentions to "exterminate" all lifeforms in the Amatria galaxy. The Spherium soon responded with intentions to war and declared the first intergalactic war.
The first fleet, built by the Spherium, arrived in Artria and attacked a major colony world, destroying all cities on the planet. The fleet soon went under radar until they approached the galactic center where most colonies of the Republic were found. Upon their discovery, the colonies were overwhelmed and completely eliminated within less than a week.
Until the Republic could respond, they built larger fleets from all the resources they could use to build the warships. The fleets were equiped with Wormhole Drives that allowed them to travel between galaxies. The Republic fleets arrived in Amatria and also attacked a major colony world, destroying the cities and killing the citizens.
It lasted for eleven years when the two governments signed the Juline Treaty, leading to the end of the war. Even after the war ended, the Artrian Galactic Republic had launched a series of terrorist attacks starting in 9997 with the Attack on Heria. Since then, both galaxies have entered an intergalactic cold war.