"Half the battle"

[Dan Goncharoff]

One of the Super Bowl commercials, the one for Jeep about troops returning
home, had the following line:

"Half the battle is knowing that this is half the battle."

http://www.youtube.com/watch?feature=player_embedded&v=FadwTBcvISo

My brain promptly exploded, but when I had put it back together again, I
wondered whether we know the origin of "half the battle" as a phrase.
Anyone?

DanG

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