It is safe to say that inbreeding led to the collapse of the tsardom in Russia.
Tsar Nikolai II had married the granddaughter of queen Victoria and thus it was no surprise that Nikolai’s son had hemophilia.
This made it easy for the mystic “healer” Rasputin to start dominating the court which in turn destroyed the trust people had in the Tsar. Soon he was deposed.
All this because royals tended to be inbred.