In the last sentence of the passage, Churchill compares the situation facing Britain to a "dread balance sheet." This analogy is similar to a bookkeeper's balance sheet, which is a summary of a company's assets and liabilities, and is used to assess its financial health.
Churchill's comparison serves to illustrate the gravity of the situation facing Britain, while also highlighting the importance of taking a clear-eyed view of the risks and challenges ahead. He acknowledges that the balance sheet is a "dread" one, meaning that it contains a number of serious threats and dangers, including the possibility of invasion and the ongoing strain of the war effort.
At the same time, however, Churchill is careful to note that this is not a time for panic or despair. Instead, he stresses the need for "intense vigilance and exertion," drawing on the lessons learned during the war thus far, and the confidence gained from Britain's ability to withstand the Nazi onslaught.
Overall, the comparison to a balance sheet helps Churchill to frame the situation facing Britain in terms that are both stark and familiar. By using a metaphor that resonates with people's everyday experience of assessing financial risk and opportunity, he is able to convey the seriousness of the challenges ahead while also instilling a sense of resolve and determination to see the war through to a successful conclusion.