What historical event necessitated outcrossing programs for the Russian Blue breed after World War II?

Answer

Low numbers of the breed

Due to a low population count following World War II, breeders in England and Scandinavia introduced outcrossing, using breeds like the blue British Shorthair or blue-point Siamese to preserve the type.

What historical event necessitated outcrossing programs for the Russian Blue breed after World War II?
animalÇatbreedRussian Blue