An invasion of armies can be resisted; an invasion of ideas cannot be resisted.

[On résiste à l’invasion des armées ; on ne résiste pas à l’invasion des idées.]

Victor Hugo (1802–1885) French writer, journalist, human rights activist, politician

The History of a Crime [Histoire d’un Crime], ch. 10, Conclusion [tr. Joyce & Locker (1878)]
    (Source)

Garson O'Toole, Burton Stevenson, and Ralph Keyes suggest this phrase morphed in English in the early 1940s into "One cannot resist an idea whose time has come," which is also widely attributed to Hugo. For more discussion about this quotation, this variation, and more, see:

• Quote Origin: Nothing Is More Powerful Than an Idea Whose Time Has Come – Quote Investigator®.
• Stevensons Book Of Quotations 4th Edition: Burton Stevenson.
• The Quote Verifier: Ralph Keyes

(Source (French)). Other translations:

One resists the invasion of armies; one does not resist the invasion of ideas.
[tr. Atheneum Society (1878)]

An invasion of armies can be resisted, but there is no resistance to an invasion of ideas.
[tr. Smith (1888)]

One can resist the invasion of armies, but not the invasion of ideas.
[Source (1900)]

One withstands the invasion of armies; one does not withstand the invasion of ideas.
[E.g. (2012)]

One can resist the invasion of armies; one cannot resist the invasion of ideas.
[E.g. (2021)]