Randall Wallace, the screenwriter of Pearl Harbor, readily admitted that he copied the line from Tora! Tora! Tora!. The director of the movie Tora! Tora! Tora!, Richard Fleischer, stated that while Yamamoto may never have said those words, the film's producer, Elmo Williams, had found the line written in Yamamoto's diary. Williams, in turn, has stated that Larry Forrester, the screenwriter, found a 1943 letter from Yamamoto to the Admiralty in Tokyo containing the quotation. However, Forrester cannot produce the letter, nor can anyone else, American or Japanese, recall or find it.
In "The Reluctant Admiral," Hiroyuki Agawa, without a citation, does give a quotation from a reply by Admiral Yamamoto to Ogata Taketora on January 9, 1942, which is strikingly similar to the famous version: "A military man can scarcely pride himself on having 'smitten a sleeping enemy'; it is more a matter of shame, simply, for the one smitten. I would rather you made your appraisal after seeing what the enemy does, since it is certain that, angered and outraged, he will soon launch a determined counterattack."
Yamamoto believed that Japan could not win a protracted war with the United States, and moreover seems to have believed that the Pearl Harbor attack had become a blunder — even though he was the person who came up with the idea of a surprise attack. The Reluctant Admiral relates that "Yamamoto alone" (while all his staff members were celebrating) spent the day after Pearl Harbor "sunk in apparent depression." He is also known to have been upset by the bungling of the Foreign Ministry which led to the attack happening while the countries were technically at peace, thus making the incident an unprovoked sneak attack that would certainly enrage the enemy.
The line serves as a dramatic ending to the attack, and may well have encapsulated some of his real feelings about it, but it has yet to be verified. After the war, a similar rumor disseminated among Occupation insiders that upon learning the attack had been a success, Admiral Yamamoto had said to those around him, "Gentlemen, we have just kicked a rabid dog." (This would have been a tactical metaphor and not intended as an insult, since he was generally fond of America and Americans.)
The other common Yamamoto quotation predicting the future outcome of a naval war against the United States ("I can run wild for six months … after that, I have no expectation of success.") is real, and is something he is recorded to have said to a number of different Cabinet members in Japan in the 1940 time period. As it happened, the Battle of Midway, the critical naval battle considered to be the turning point of the war, indeed did occur six months after Pearl Harbor (Midway ended on June 7th, exactly 6 months later).
Similar to the above quotation was another quotation that, while real, was widely misinterpreted in the US press. Yamamoto, when once asked his opinion on the war, pessimistically said that the only way for Japan to win the war was to dictate terms in the White House, requiring them to eventually invade the United States and march across the country while fighting their way to Washington — i.e., Japan would have to conquer the whole of the United States. Yamamoto's meaning was that military victory, in a protracted war against an opponent with as much of a population and industrial advantage as the United States possessed, was completely impossible--a rebuff to those who thought that winning a major battle against the US Navy would end the war. However, in the US, his words were recast as a jingoistic boast that he would in fact dictate peace terms at the White House.