During a flashback to the last time Steppenwolf and his Parademon armies attempted to take over the Earth, we learn that only a union of humans, gods, Amazons, Atlanteans and aliens was able to stop them. Specifically, those aliens were members of the Green Lantern Corps.
Yes, after months and months of speculation about whether Hal Jordan and John Stewart would be appearing in Justice League, it turns out that they don’t, but some generic alien Corpsmen do instead. Though not named in the film, any DC fan will recognize the characters from their green outfits and Power Rings.
Noticeably, one unfortunate Lantern is slain by Steppenwolf in battle. The general then watches as the ring slides from the warrior’s finger and floats off into space. This serves as a handy reminder of how new Green Lanterns are chosen. Such as Abin Sur’s ring seeking out Hal Jordan after its previous master’s death.