The Northern Cross War was the first war against Bacterian.
In 6809, Venom and his fleet warped back to January 6644. They did this because they wanted 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.
The war somehow left amounts of radiation on the southern hemisphere of Gradius, killing most of the Wreek that lived there. So in essence, It was Dr. Venom who caused the death of his own race by starting the war.