Arrange the steps of the argument that period labels shape historical explanation.

1. A historian chooses a date at which a period begins
2. That choice asserts which change was decisive
3. Events before the boundary become background rather than cause
4. The narrative built inside the period inherits that assumption
5. A rival periodisation would make different events look causal

The third step is where the damage is done quietly. Once something is pushed before the boundary it stops being a candidate explanation and becomes context, which is how a periodisation ends up producing exactly the causal story it assumed, with no dishonesty required at any point.