A fish rots from the head down
When an organization or state fails, it is the leadership that is the root cause.
This proverb is of ancient origin but precisely which of the ancients coined it is probably beyond our ken at this distant remove.
Many countries lay claim to it. I've seen sources that place it in China, Russia, Poland, England, Greece and so on, but usually with no evidence to substantiate those claims. A correspondent of mine has asserted that it was written in a Greek text by Erasmus, who died in 1546. That may be the case, but I've not been able to substantiate that claim.
Actually, they don't.
All of the early examples of the phrase in print in English prefer the variant 'a fish stinks from the head down' to 'a fish rots from the head down', which is more popular nowadays. Those early examples all ignore the nations mentioned above and credit the term to the Turks. Sir James Porter's Observations on the religion, law, government, and manners of the Turks, 1768, includes this:
The Turks have a homely proverb applied on such occasions: they say "the fish stinks first at the head", meaning, that if the servant is disorderly, it is because the master is so.
The early date of this citation and the fact that Porter was in a position to be authoritative on Turkish custom, being British ambassador to the Sublime Porte of the Ottoman Empire for 15 years in the second half of the 18th century, gives Turkey a strong claim to be the birthplace of this proverb.
Of course, the proverb isn't a lesson in piscine biology. The phrase appears to have been used in Turkey in a metaphorical rather than literal sense from the outset. That's just as well as, in reality, it is the guts of fish that rot and stink before the head.