Justice League

In 1962's Justice League of America #9 Earth was infiltrated by the Appelaxians, competing alien warriors sent to see who could conquer Earth first to determine who will become the new ruler of their home planet. The aliens' attacks drew the attentions of Superman, Batman, Wonder Woman, the Flash, Green Lantern, Aquaman, and the Martian Manhunter. While the superheroes individually defeated most of the invaders, the heroes fell prey to a single competitor's attack; only by working together were they able to defeat the competitor. For many years, the heroes heralded this adventure as the event that prompted them to agree to pool resources when confronted with similar menaces.