The corruption of each government almost always begins with that of its principles.

[La corruption de chaque government commence presque toujours par celle des principes.]

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

Spirit of Laws [The Spirit of the Laws; De l’esprit des lois], Book 8, ch. 1 (1748) [tr. Cohler/Miller/Stone (1989)]
    (Source)

(Source (French)). Other translations:

The corruption of each government generally begins with that of the principles.
[tr. Nugent (1750)]

The deterioration of a government begins almost always by the decay of its principles.
[ed. Hoyt (1896)]

The corruption of each government almost always begins with the corruption of the principles.
[tr. Stewart (2018)