The quote "We have met the enemy and he is us" is a parody of a message sent in 1813 from U.S. Navy Commodore Oliver Hazard Perry to Army General William Henry Harrison after his victory in the Battle of Lake Erie, stating, "We have met the enemy, and they are ours." It first appeared in a lengthier form in A Word to the Fore, the foreword of the book The Pogo Papers, first published in 1953. Since the strips reprinted in Papers included the first appearances of Mole and Simple J. Malarkey, beginning Kelly's attacks on McCarthyism, Kelly used the foreword to defend his actions:
Traces of nobility, gentleness and courage persist in all people, do what we will to stamp out the trend. So, too, do those characteristics which are ugly. It is just unfortunate that in the clumsy hands of a cartoonist all traits become ridiculous, leading to a certain amount of self-conscious expostulation and the desire to join battle. There is no need to sally forth, for it remains true that those things which make us human are, curiously enough, always close at hand. Resolve then, that on this very ground, with small flags waving and tinny blasts on tiny trumpets, we shall meet the enemy, and not only may he be ours, he may be us. Forward! (Walt Kelly, June 1953)
The finalized version of the quotation appeared in a 1970 anti-pollution poster for Earth Day (see above) and was repeated a year later in the daily strip.
—from the Wikipedia entry on Pogo (comic strip)