"But then, if the war continues after that, I have no expectation of success"

Isoroku Yamamoto's quote shows his pessimism about the outcome of the war. He is suggesting that if the war continues, he does not think that his side will achieve success. This is likely due to the truth that his side is facing a powerful opponent and the war has already been going on for a long time. He is likely aware of the resources and workforce of the opposing side and is concerned that his side might not have the ability to match them. He is likewise most likely knowledgeable about the capacity for the war to drag on for a very long time, which might even more deplete his side's resources and spirits. His quote reflects his understanding of the situation and his lack of optimism for the future. He is likely hoping that the war will end soon, but is preparing for the worst.