Supergirl and the Legion of Super-Heroes # 29

The reason for the Dominators attack is revealed. Long ago, the Dominators assembled with other planets to take care of the potential threat of Earth. This incident is known as the Invasion. At the conclusion of the Invasion, the Dominators were beat and it left a long-lasting psychological scar on that race. Since that encounter, Earth and Dominion have had no contact. Centuries later, they have made it official with a pact called the Non-Aggression Treaty. However, after that treaty was place, Booster Gold arrived on the Dominators homeworld to steal a weapon. This occurred during the events of 52, and the Dominators took that to mean that there were 52 planets set up to attack the Dominators. This prompted them to attack the satellite planets and outposts of the United Planets and their eventual attack on Earth.