Trace how social identity theory gets from a harmless act of categorisation to intergroup conflict. Put the steps in causal order.

1. People are categorised, however trivially, into an in-group and an out-group.
2. Membership of the in-group becomes part of each member's self-concept.
3. Members are motivated to see the in-group favourably, because it now reflects on the self.
4. The in-group is favoured while the out-group is downgraded and seen as all alike.
5. Comparison hardens into competition, and conflict between the groups becomes easy to ignite.

The order is the argument: categorisation feeds identity, identity supplies the motive, the motive produces differential treatment, and differential treatment escalates into conflict. Reading it this way shows why conflict is so easy to start - the fuel is the categorisation at step 1, which costs nothing to create.