The Northern Cross War was the first war against Bacterian.
In Gradian Year 6809, Dr. Venom and his fleet warped back in time to January of Gradian Year 6644 with the objective to kill James Burton so that they can change the future. The war ended in July 6644 when David Burton, a descendant of James, eliminated Venom's fleet, rescued James and returned the three-year old child back to Gradius before departing and warping back to his time.
The war somehow left high amounts of radiation on the southern hemisphere of Gradius that then killed most of the Wreek whom lived there. So in essence, it was Dr. Venom who ironically caused the death of his own race by starting the war.