Another episode of Pyun Pyun Maru. Episode #11, entitled Bakudan-chan wa Daikirai (translated as I Hate Bombs).
The story: The Circle Ninja Group created nuclear bombs (animated as cartoonish round black balls like in Western animation) to be dropped on their enemy, the Triangle Ninja Group. They entrust Pyun Pyun Maru to drop two of the bombs, much to their objections. After they drop one of the bombs on the enemy early (by accident), they decide to disguise the second bomb as a watermelon. Mix-ups ensues.
Japan is very anti-nuclear for obvious reasons, so it's interesting that there was an anime where a group of ninjas develop such weapons, but based on the context, it's clear that it was written with the idea that it's not a good idea. Look at the episode title, for goodness sake.
When the first bomb is dropped, the camera pans to the aftermath of the explosion, where all the ninjas are knocked out. In the background you can see a dome that was destroyed, even though they're in the mountain range where such structures aren't around. That dome is a reference to Genbaku Dome, a structure that was destroyed in Hiroshima when the bomb was dropped. I'm amazed that such gag was featured in a 1967 Japanese cartoon. For context, when this was broadcast 22 years have gone by when the atomic bombs were dropped in Hiroshima and Nagasaki.
Would a similar joke with the World Trade Center occur in American cartoons made 20+ years after 9/11?
Aired September 11, 1967 on NET (Nihon Educational Television)
Written by Toyohiro Andô
Directed by Yoshio Takami
Animation Director: Keiichiro Kimura