He probably had planned a way to work Pearl Harbor into the presser somehow. I doubt it was just off the cuff. He loves to put his guests on the spot like that.
Also, this was pretty far from a surprise attack. Maybe he didn't give allied countries a courtesy call right before, but you can't build up like that and be like "surprise, I bet no one saw that coming".
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"A pessimist thinks things can't get any worse. An optimist knows they can."
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