An empire founded by arms needs to be sustained by arms.

[Un empire fondé par les armes a besoin de se soutenir par les armes.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Considerations on the Causes of the Greatness of the Romans and Their Decline [Considérations sur les causes de la grandeur des Romains et de leur décadence], ch. 18 (1734, 1748 ed.) [tr. Baker (1882)]
    (Source)

(Source (French)). Other translations:

An empire, founded by arms, must likewise have arms for its support.
[tr. B--- (1734)]

An Empire founded by Arms, must likewise have Arms for its Support.
[ed. Guterman (1963)]

An empire founded by war has to maintain itself by war.
[E.g. (1960)]

An empire founded by arms needs to be sustained by arms.
[tr. Lowenthal (1965)]