Summary:

He planned to bring a group of boys together, allow them to make friends, then separate them into two factions to compete for a prize. He believed they would forget their friendships and start demonizing one another at this point. The piece de resistance was to come at the end: he planned to set a forest fire in the vicinity of the camp. The intentional forest fire would force the boys to work as one team again, facing a shared threat.

Sources:

Book: the Lord of the Flies by Gina Perry 

Robbers Cave Experiment: How Group Conflicts Develop - PsyBlog (spring.org.uk)

Robbers Cave Experiment (SOCIAL PSYCHOLOGY) - iResearchNet

Robbers Cave Experiment / Realistic Conflict Theory | Simply Psychology

A real-life Lord of the Flies: the troubling legacy of the Robbers Cave experiment | Psychology | The Guardian

‘Come on out and fight!’: an extract from The Lost Boys by Gina Perry | Psychology | The Guardian

The Robbers Cave Experiment: Realistic Conflict Theory - Explore Psychology