[nickcarraway]

No, what he meant by, “soft underbelly,” is, it was economically in the best interest of the British Empire to occupy Sicily.

[Billthedrill]

We'll have to agree to disagree there. Churchill detailed some of his actual correspondence in Closing The Ring, which is (IMHO) one of the best of his epochal Second World War series. Selectively, to be sure, but it's primary source. You can't do much better than that.