Isoroku Yamamoto; Wikimedia Commons

Isoroku Yamamoto; Wikimedia Commons

After the Pearl Harbor attack, it has often been claimed that its chief architect Admiral Isoroku Yamamoto, Commander-in-chief of Imperial Japan’s Combined Fleet during World War II, uttered:
“I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve.”
Yamamoto studied at Harvard University (1919–1921) and was twice posted as a naval attaché in Washington, D.C. before the war. Fluent in English, he traveled extensively in the USA, studying customs and business practices. Yamamoto was no doubt familiar with potential American industrial might, and might well have had these thoughts.
However, there is no reliable documentation that he ever actually said this. 
Nevertheless, that appears to be exactly what happened after mid-1942.