The most likely reason why Churchill regularly brings up negative details about the war is that he wants the British people to understand both the negative and the positive aspects of Britain’s war-readiness. If the people are too positive, they might get too discouraged when things start to go wrong. They might also have unrealistic expectations, or they might believe they do not need to fight as hard. Therefore, by mentioning both the positive and the negative details, it is more likely they will be prepared for anything.
