Answer:
Keep calm and carry on
Step-by-step explanation:
The main theme of a narrative is related to the main subject that the literary work addresses. In "The evacuation", we can see that the narrative revolves around continuing to fight for what you want most at the moment, even if the expectations and chances are tiny and almost nonexistent. In this situation it is best to remain calm and think positively.
Based on that, we can say that the main theme of "The evacuation" is closely related to the theme "Keep calm and carry on".