“When you have come to grips and are striving together with the enemy, and you realise that you cannot advance, you "soak in" and become one with the enemy. You can win by applying a suitable technique while you are mutually entangled.
In battles involving large numbers as well as in fights with small numbers, you can often win decisively with the advantage of knowing how to "soak" into the enemy, whereas, were you to draw apart, you would lose the chance to win. Research this well.”

Variant translations
If you know others and know yourself, you will not be imperiled in a hundred battles; if you do not know others but know yourself, you win one and lose one; if you do not know others and do not know yourself, you will be imperiled in every single battle.
Know your enemy and know yourself, find naught in fear for 100 battles. Know yourself but not your enemy, find level of loss and victory. Know not thy enemy nor yourself, wallow in defeat every time.
Literal translation: Know [the] other, know [the] self, hundred battles without danger; not knowing [the] other but know [the] self, one win one loss; not knowing [the] other, not knowing [the] self, every battle must [be] lost.
Source: The Art of War, Chapter III · Strategic Attack

Source: Short fiction, Hardfought (1983), p. 69

Go Rin No Sho (1645), The Fire Book

Prayers For The Assassin (2006)

To Leon Goldensohn, February 4, 1946, from "The Nuremberg Interviews" by Leon Goldensohn, Robert Gellately - History - 2004.

Attributed by Gavin McInnes (AKA Miles McInnes) of Rebel Media in the satirical video "Why Miles McInnes is moving back to Canada: Trudeau's 10 Greatest Quotes!" at 5m45s number 9/10 https://www.youtube.com/watch?v=_tpW1m-8yVQ&t=5m45s", along with accompanying article as Gavin McInnes “If you kill your enemies, they win”: Justin Trudeau’s greatest quotes! https://www.therebel.media/justin_trudeau_s_greatest_quotes released the same day, then repeated by Daniel Greenfield in his article "Surrendering to ISIS is the Only Way to Defeat It", Canada Free Press (31 March 2016) https://canadafreepress.com/article/surrendering-to-isis-is-the-only-way-to-defeat-it. A Reddit thread "Any evidence that Trudeau said 'if you kill your enemies, they win'?" https://www.reddit.com/r/canada/comments/68iwe3/any_evidence_that_trudeau_said_if_you_kill_your/ questioning the veracity of the attribution was created in April 2017. No corroborating evidence was found to support the "McCinnes brothers" claims that Trudeau said this, yet many others have repeated it as fact. It is possibly a paraphrasing of Trudeau's February 2016 quote about elevating enemies.
Misattributed

Classified Appendix (p. 354)
The Laundry Files, The Apocalypse Codex (2012)

Misattributed