Spoilers for Justice League are in this post! Don't read if you haven't seen the film yet.
With the recent release of Justice League, I’ve had a lot of thoughts about Justice League and Batman vs Superman and how they can’t be dismissed lightly because of how layered the films and the overall story are:
First of all, we can’t take the plot of these films too lightly. If you think Justice League is about the Justice League defeating the alien threat of Steppenwolf and his parademons, then you should go rewatch the film again (I say rewatch it anyways, cause it’s such a great film), as this is the secondary plot of the movie. The primary plot of Justice League is really about the formation of the team and the individual team members having to learn how to come together and unite to form an unbeatable force (it’s all right in front of us in the marketing of the film). They all have a personal journey to make, and they have to overcome their fears and doubts before they can begin to trust each other and work together. This is illustrated to us very well in the beginnings of the film as Bruce Wayne and Diana Prince contact the different metahumans, Barry Allen, Arthur Curry, and Victor Stone. The names of these three and others were given to Bruce by Amanda Waller as shown to us in the Oscar winning film - Suicide Squad, which occurred in between Dawn of Justice and Justice League.
We are also shown how invincible Steppenwolf is as he is able to easily overcome the individual forces, the Amazonians, the Atlanteans, and Man to obtain the Motherboxes. It isn’t until the League is able to unite and work together that they are able to defeat him, and even then they need some “super” help (sorry, couldn’t help myself. lol). We see how, without Superman, Earth is doomed and the inhabitants of Earth would be unable to defeat Steppenwolf. The League has to learn how to work together to bring Superman back and they all played a part in bringing him back with the help of one of the Motherboxes. It isn’t until Superman is fully back and unites with the League that Steppenwolf can be defeated and sent packing.
Some have thought that Batman vs Superman: Dawn of Justice only showed us 15 mins (or whatever it ended up being) of Superman and Batman actually fighting, if so - then you need to rewatch the film. Batman versus Superman starts in the Prologue of the film when Batman witnesses firsthand the destruction of Metropolis and he sees Superman and Zod destroying Metropolis and he decides Superman is dangerous and he then starts his quest to destroy Superman. The whole movie is Batman/Bruce Wayne’s machinations to destroy Superman.
Having Lex Luthor behind the scenes pulling the strings to drag Batman further down into the darkness is icing on the cake for this film. While Batman is distracted by figuring out a way to defeat Superman, the real villain, who is Lex, ultimately unleashes a devil to destroy a god, in which he eventually succeeds. Lex is aware that in defeating Superman he can bring in true chaos to the world by way of the Motherboxes. The effects of Lex's plan is dealt with in Justice League, which in turn makes that last end credit scene in Justice League more meaningful and it isn't just eye candy.
Lex would’ve triumphed except for he forgot the one thing that trumps everything and can’t be quantified- Love. Especially the love a son has for his mother, and how some men would do anything to protect their mother. He was counting on Clark Kent's love for his mother to manipulate him into defeating Batman, but he overlooked Bruce's love for his long dead mother. Maybe he thought Bruce was too far down the dark path and couldn't be redeemed because of the crimes he had committed. Bruce Wayne became Batman because of the love he had for his mother and father. But he had lost sight of this over the past couple of years due to the darkness of Gotham. He was becoming the type of person he had initially set out to protect people from. Clark’s love for his mother broke through this darkness and reminded Bruce of why he became Batman in the first place. He became Batman to protect the Martha and Thomas Wayne’s of the world.
With Clark Kent speaking the name of “Martha” and to "save Martha" this snapped Bruce back to what he was all about in the first place and he also realized the being in front of him was more like him than he realized. He saw a chance to do what his pre-teen self couldn't do - "save Martha". He saw that Clark cared just as much for his mother and that his last dying breath was for someone to save his mother regardless of what happened to him. This is why he was willing to spare Superman and go save Martha Kent while Clark went to deal with Lex. Thus the beginning of the Justice League and you can say the Dawn of Justice.