"Fortresses are most easily breached from within." This saying has different origins and references in different cultural and historical contexts. The following are some of the possible sources:

1. Sun Wu, the ancient Chinese military strategist, said in his book "The Art of War": "The method of attacking a city is to use it as a last resort. It will be completed after a month, and it will be three months away from the ground. The ants will attack it; rush into it, burn its accumulated firepower, and then attack the city. Wait for the expense. The advantage is to attack from the outside."

2. The British writer and social critic George Orwell said in his work "Animal Farm". ), in which the animals thought they were overthrowing humans, but in fact they were deceived and controlled by the pig leadership.

3. Russian proverb: "Как и всякая крепость, враги подкрадываются изнутри." (Like every castle, the enemy sneaks in from the inside.) This is a common Russian proverb, which means that a fortress often comes from within. The interior was breached.

In summary, the implication of this quote is that even if an organization or structure seems impenetrable, it can become fragile and vulnerable if something goes wrong internally.