Vandal Savage created an alternate timeline by interfering with the events of World War II. In this alternate timeline, Bruce Wayne's parents were killed for speaking out against Savage's brutal regime. Thus, Bruce became Batman, the leader of the resistance movement against the regime.
When the Justice League from the original timeline appeared on Earth, Batman helped them escape from the authorities and learned of their timeline. He assisted the League in breaking into the facility where the time portal had appeared. However, Batman did not accompany the League into the past to thwart Savage; he preferred to stay behind and continue to run the resistance in case the League failed.