Zou no Inai Doubutsuen

After the war, concerned brother Hide promises his critically ill sister Miyoko that he will arrange for her to see a real elephant. But when he goes to the zoo to find one, an old man tells him of the tragic day when the zoo animals were slaughtered.

The story is based on true historical events of the Ueno Zoo in Tokyo, where dangerous animals during WW2 were slaughtered in order to prevent them from wreaking havoc in case Allied bombings would let the animals escape. Various methods were used such as starvation, poisoning, death by firing squad, and decapitation. Tokyo was bombed over a year later after the animals' slaughter and Ueno Zoo was never hit.

(Source: The Anime Encyclopedia and Wikipedia)