Faux-quotations are almost always without context unread by the poster.
http://www.factcheck.org/2009/05/misquoting-yamamoto/You cannot invade the mainland United States. There would be a rifle behind every blade of grass.
It has been declared this attribution is "unsubstantiated and almost certainly bogus, even though it has been repeated thousands of times in various Internet postings. There is no record of the commander in chief of Japanís wartime fleet ever saying it.", according to Brooks Jackson in "Misquoting Yamamoto" at Factcheck.org (11 May 2009), which cites Donald M. Goldstein, sometimes called "the dean of Pearl Harbor historians", writing "I have never seen it in writing. It has been attributed to the Prange files [the files of the late Gordon W. Prange, chief historian on the staff of Gen. Douglas MacArthur]." http://en.wikiquote.org/wiki/Yamamoto,_Isoroku