The Human-God war started in 3983 and has continued into the early to id 4120's. This century long spanning war began when Humanity discovered a great number of wormhole disturbances towards the Obscure, the realm of the Milky way out of view from Earth.
No human alive remembers a time when war did not encompass almost every aspect of daily life. The War, or "The Slog" as described by intercepted transmissions from the God races, turned from a deadly invasion of the Milky Way into a grueling battle of attrition that has plagued the front lines for generations.
The God Invasion lead to the deaths of billions and the loss of hundreds of worlds in just a few short years. At the turn of forty first century the CMC finally slowed the invasion with the creation of new fleets and armies. Since then the war has caused the deaths of trillions of humans and has left thousands of previously Human controlled planets as inhospitable balls of dust. With several hundred planets reduced to vast asteroid fields.
To this day there is no definitive reason as to why the Gods attacked, although many theories do exist. Any and all attempts at communication from Humanity have been met with silence or retaliation, even from those in Humanity who have pledged allegiance to the invaders.