In the nascent stages of the DC film’s life, there was going to be a moment in which The Caped Crusader got his hands on a massive piece of parademon artillery to use against the invading forces — presumably in the final Russia sequence.
Though some of Zack Snyder’s Vero posts have shown fully-shot sequences from Justice League that didn’t make it into the final cut, this particular comment on a fan post seems to reveal something far earlier in Justice League’s development.
Batman is the greatest tactician in the DC mythos, so it would make quite a bit of sense for him to want to get his hands on something with the ability to turn the tide in such a substantial way.