You cannot invade the mainland United States. There would be a rifle behind every blade of grass.

Some have declared this attribution is "unsubstantiated” even though it has been repeated thousands of times in various Internet postings. The quote is supposed to be in a letter, but the letter hasn’t been found or no longer exists. There is no record found yet of the commander in chief of Japan’s wartime fleet ever saying it. The jury is still out on this.