The Thanagarians were a race of extraterrestrial beings that invaded Earth in 2175.
When Vandal Savage did not unite Earth, the Thanagarians destroyed the world. 
After the destruction of Oculus the Time Masters were unable to calculate the future of the current timeline, citing the Thanagarian invasion as a possibility that may or may not happen. Due to this change of plans, the Time Masters withdrew their support of Savage's cause. 
Behind the scenes
- In DC Comics the Thanagarians were the population of planet Thanagar. They were a race of bird-winged people, with their traditional attire resembling Hawkman suits. The Thanagarians were aggressive and dreamed of turning the entire universe into a fascist police state mirroring their own.
- The idea of an invasion of the Earth by Thanagarians, could be inspired by the episode of the Justice League animated tv series named Starcrossed (which is in fact the three-episode-finale of the season 2).