Got it. Ring for Jeeves, p. 141 of the Overlook edition. Jeeves and Bill Belfry, Ninth Earl of Rowcester:

"Were you in the First World War, Jeeves?"
"I dabbled in it to a certain extent, m'lord."
"I missed that one because I wasn't born, but I was in the Commandos in this last one. This is rather like waiting for zero hour, isn't it?"
"The sensation is not dissimilar, m'lord."