What year was that published? I read the message as "support the US war effort so that we don't get conquored by our enemies". That doesn't seem like a bad message to me.
The main problem I can see is the depiction of the Japanese person (the emporor?). That's pretty offensive by modern standards, but I don't see a reason we should apply those standards here.
Even today, cartoonists regularly exaggerate facial features of almost anyone to serve their purposes, so if you think that's wrong you simply don't like the medium. And we were at war, do you expect him to show reverence for the foreign leaders who are making war against his country?
Strange to think that the caricature is quickly dying. It was a crux of political cartoons throughout all of the 1900's, and also a common form of street art.
You didn't take the effort to explain the relevance.
E.g., you could have said that you agree with the message but that you didn't want this piece to stand as a kind of Dr. Seuss image rehabilitation project that serves partly to whitewash his legacy in the interests of his current estate holders.
On the other hand, you could merely be trolling in an attempt to derail the topic of the book.
I like to keep track of functional equivalencies in cases like this. So if for whatever reason someone doesn't want to discuss the fantastic Butter Battle Book, we could just as easily discuss H. Bruce Franklin's non-fiction War Stars: The Superweapon and the American Imagination. He traces weapons dev from Robert Fulton hawking one of the first submarines all the way to nuclear weapons. (Plus it looks like it was updated to include 21st century weapons development.)
The main problem I can see is the depiction of the Japanese person (the emporor?). That's pretty offensive by modern standards, but I don't see a reason we should apply those standards here.
Even today, cartoonists regularly exaggerate facial features of almost anyone to serve their purposes, so if you think that's wrong you simply don't like the medium. And we were at war, do you expect him to show reverence for the foreign leaders who are making war against his country?