Apocalypse and his horsemen are teleported into Metropolis after a Celestial tech experiment.
Round 1: Someone calls the police and this is the league available.
Round 2: Apoc and the others manage to stay out of sight. Over the course of the year the groups have time to study each other without a direct confrontation and time to prep. (no sundip, sinister has captured many mutants/metahumans to study)