Actually, it might be.
You're right that it seems much older than "The Godfather: Part II" — it was actually said in the sequel, by the new Godfather, Al Pacino's ("Scarface," 1983) Michael Corleone. The movie was released in 1974, a mere 47 years ago.
I can say "mere" because a lot of people credit it to ancient sources such as Sun Tzu's "The Art of War" (a work of military strategy written sometime around the sixth century BCE) and Nicolo Machiavelli's "The Prince" (written about 1513). Yet close reads of those two works don't turn up exact matches.
Sun Tzu said a number of things relating to knowing your enemy (most notably, "If you know the enemy and know yourself, you need not fear the result of a hundred battles").
Meanwhile, Machiavelli came closer to Corleone's sentiment, though wordier: "It is easier for the prince to make friends of those men who were contented under the former government, and are therefore his enemies, than of those who, being discontented with it, were favourable to him and encouraged him to seize it."
Based on this evidence, it seems screenwriters Francis Ford Coppola ("Apocalypse Now," 1979) and Mario Puzo (who wrote the source novel) were drawing on a lot of military and diplomatic knowledge when crafting the phrase but made it snappy for the modern cinematic era.
Today, of course, people look to the Godfather films for insight, the same way that we look to Sun Tzu and Machiavelli. Even Tom Hanks' character in the 1998 comedy "You've Got Mail" argued that "'The Godfather' is the sum of all wisdom."
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