Question

The Spartans held Athens and Thebes, establishing there an oligarchy: nevertheless they lost them. The Romans, in order to hold Capua, Carthage, and Numantia, dismantled them, and did not lose them. They wished to hold Greece as the Spartans held it, making it free and permitting its laws, and did not succeed. So to hold it they were compelled to dismantle many cities in the country, for in truth there is no safe way to retain them otherwise than by ruining them. And he who becomes master of a city accustomed to freedom and does not destroy it, may expect to be destroyed by it.

–The Prince,
Niccolò Machiavelli

Which detail best supports the idea that princes who do not destroy or occupy their principalities will not keep them?

“The Spartans held Athens and Thebes, establishing there an oligarchy.”
“The Romans, in order to hold Capua, Carthage, and Numantia, dismantled them.”
“They wished to hold Greece as the Spartans held it, making it free and permitting its laws.”
“He who becomes master of a city accustomed to freedom and does not destroy it, may expect to be destroyed by it.”

Answer 1

“He who becomes master of a city accustomed to freedom and does not destroy it, may expect to be destroyed by it.”

This sentence directly states the idea that princes who fail to destroy or occupy their principalities will not keep them.