Justice League introduced the moviegoing world to The Flash, Cyborg, and Aquaman, but arguably its biggest impact on the DC Extended Universe was the way it reset Ben Affleck's Batman.
In Batman v. Superman: Dawn of Justice, the Dark Knight Detective was a grumbling, brooding borderline sociopath who set out to murder the Man of Steel based on paranoia. By the time Justice League rolls around, Batman has realized that his previous actions were foolish, and that the only way to save a world facing the kind of stakes seen in Man of Steel is to rally together with larger-than-life heroes, rather than opposing them.
The end of Batman v Superman was about Bruce Wayne realizing that he was not infallible, and that he needed to learn to trust in people who had done nothing to earn his enmity. That is a lesson he carried through to Justice League, and one of the most interesting scenes in the film is actually one that does a kind of cracked-mirror reflection of the comics.
In an issue of the 2005 event miniseries Infinite Crisis, Batman is arguing with Superman and Wonder Woman when he tells the Man of Steel "the last time you inspired anybody was when you were dead." The insult was so cutting, and so absurd in some ways, that it became a popular internet meme for DC fans.
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