With the defeat of the allied forces on D-Day in World War 2, Nazi Germany won the war thanks to the leadership of Vandal Savage, Following the war, Savage proceeded to conquer the majority of the known world; including the United States of America.
When his parents were executed for speaking out against the regime, Bruce Wayne started the resistance; recruiting like-minded orphans and outsiders to battle against Vandal Savage's Nazi regime from underground in Metropolis.
Cassandra was one of these resistance fighters recruited and trained by Bruce to aid the cause of freedom from Savage's dictatorship.
Following the arrival of the Justice League and the discovery of a prototype worm-hole used to travel through time, the heroes used the worm-hole to travel back to D-Day and ensure that the allied forces won the battle; resulting in history returning to its initial course and the irradication of this timeline.