This isn't really true as far as I am aware. If you are getting this from the Robber's Cave Experiment(HPMOR), it was misrepresented there. They were only hostile after it was induced and they went back to being nice in the last stage too.

Indeed - I've heard it referred to as 'ingroup/outgroup behaviour' (https://en.wikipedia.org/wiki/Ingroups_and_outgroups)