Have you read “The River War?” I got it from Amazon and it was a print-to-order version and omitted that quote. I think it was Churchill’s first book.
No I have not read The River War, but that quote has been repeated numerous times here on FR and elsewhere.
It's also (still) possible to verify that the quote is in fact due to Churchill in The River War, although I won't be surprised if it scrubbed from the internet at some point.
You can easily Google it and find the quote at these sources, among others:
Quotes from book The River War
Winston Churchill - The River War (1899)
That the passage was not present in the version you got from Amazon is — at the very least — interesting.
The book is available free on line from several sources.