Final answer:
The emotion that Auden believes people must experience after September 1, 1939 is sadness.
Step-by-step explanation:
In the poem 'September 1, 1939,', the poet W.H. Auden believes that people must experience the emotion of sadness after the events of September 1, 1939. The poem reflects on the outbreak of World War II and the darkness and despair that followed. Auden argues that only through acknowledging and experiencing this sadness can people come together in empathy and work towards a better future.