The quote you are most likely thinking of is

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

The quote was allegedly said by Japanese Admiral and planner of the Pearl Harbor attack, Isoroku Yamamoto. Yamaoto is notorious for being misquoted. People also often attribute this quote

I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve.

to him yet, he never actually said it.

The quote was most likely said by an American officer serving in the Pacific. Daniel Goldstein, a notable historian of the Pacific War said this about the quote

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] but no one had ever seen it or cited it from where they got it. Some people say that it came from our work but I never said it. … As of today it is bogus until someone can cite when and where.