Is the idea of a known plaintext attack really that new? As you can probably guess, I just watched The Imitation Game, which hasn't really gotten points for historical accuracy...

During World War II, Turing and his team fret that they can't know the plaintext of the Enigma messages they intercept. In a Eureka moment, it occurs to Turing to try a message that probably contains the words "Heil Hitler."

My question is then: Hadn't it occurred to anyone in the past, say, during the Civil War or World War I, to try something like this to break a code? (I might be mixing up known plaintext and chosen plaintext attacks, please let me know).
No, the idea is much older. Don't trust the movie too far, Turing actually searched for the word "eins" (one) because it was the most common number used. Far more likely to appear than "Heil Hitler" but it doesn't make good screen drama.