A prince never lacks legitimate reasons to break his promise.
Niccolo Machiavelli.
If possible try to read his The Prince.
I also have tried. Never completed. But its a masterpiece. A real political theory. He was like Chanakya of west. You many a time disagree with of ethical basis, but at the core of the heart you give always positive thoughts to his sayings and ways. Understanding of the system.
And one more thing. He lived in then Florence, circa 1500s. Thats something which makes me jealous.
As a reminder, source of the word Machiavellian, now coming into meaning as shrewd, cunning and doing all for reaching his goals.
P.S. This meaning is not there in Word Web