Answer:
Invasion from another country.
Step-by-step explanation:
When a country is very powerful, and suddenly power begins to shift, there are some concerns that the country might have about this change of circumstances. One of these concerns would be the fact that a different country could become powerful enough to invade the former one. This is a fear Great Britain most likely had due to the rise of countries like Germany. Therefore, the idea of an alien invasion plays on these fears and would most likely create an emotional response.