“The enemy is us.”

Kriegsführung und Politik (Berlin, 1922), p. 334, quoted in W. W. Coole (ed.), Thus Spake Germany (London: George Routledge & Sons, 1941), p. 114

Ch 6
The Rahotep series, Book 2: Tutankhamun

Goninan in Part One: The Hidden People, "Border Spirit" p. 336
The Little Country (1991)

This is derived from the famous statement of Commodore Oliver Hazard Perry on the "War of 1812": "We have met the enemy and they are ours". It appeared in a "modern day" poster for the first Earth Day in April 1970, and next in the comic strip itself in August 1970 in Porky Pine's mouth, and was re-used by Kelly in a subsequent Earth Day poster (1971), and further strips and in the title of the book Pogo : We Have Met the Enemy and He Is Us (1972).
A similar statement was actually used by Kelly many years earlier in his introduction http://www.igopogo.com/we_have_met.htm to The Pogo Papers (1953) which he closes with these comments:
:Traces of nobility, gentleness and courage persist in all people, do what we will to stamp out the trend. So, too, do those characteristics which are ugly. It is just unfortunate that in the clumsy hands of a cartoonist all traits become ridiculous, leading to a certain amount of self-conscious expostulation and the desire to join battle.
There is no need to sally forth, for it remains true that those things which make us human are, curiously enough, always close at hand. Resolve then, that on this very ground, with small flags waving and tinny blast on tiny trumpets, we shall meet the enemy, and not only may he be ours, he may be us.
:::Forward!
Pogo comic strip (1948 - 1975), Pogo

The Spiral Dance: A Rebirth of the Ancient Religion of the Goddess (1979)
Context: The Conqueror, whose core issue is safety splits us into Conqueror and Enemy/Victim, tells us, "Don't trust!" and generates fear, paranoia, distortions of reality, and the need to annihilate enemies. The Conqueror seduces us by making us feel special, sometimes grandiose and self-righteous, sometimes especially weak and victimized.

12 November
Without Dogma (1891)

Quoted in Engaged Buddhist Reader: Ten Years of Engaged Buddhist Publishing (1996) by Arnold Kotler, p. 106

Rien n’est si ordinaire que d’imiter ses ennemis, et d’employer leurs armes.
"Oracles" (1770)
Citas, Questions sur l'Encyclopédie (1770–1774)

To Leon Goldensohn, February 28, 1946, from "The Nuremberg Interviews" - by Leon Goldensohn, Robert Gellately - History - 2004