In the late 60's to the early 70's DC comics inserted into the stories of their successful line of war titles this Make War No More tag. Obviously, a response to the prevailing public sentiment about the Vietnam conflict.
As PSA's for comics go these are probably the ones I am most cynical about for a few reasons. The first being that the tags only showed up in the combat titles full of gleeful mayhem and nazi-smiting. I have not yet seen these in any other DC books of the era. The other reason is that everything I have read about Robert Khaniger is that he leans way to the right. So the addition of the MWNM tag only seems a shallow attempt, as a marketing ploy, to blunt the pro-war message the books had.
Also, the tag often would interfere or disturb the layout of the art, though this was mostly solved a few years later as editorial policy changed.
After a few years of the tag being present in the final panel of stories it was exiled to the letters page.
For those of you who can't get enough of Joe Kubert's combat-themed art, be sure to check out PS Magazine (a military preventive maintenance magazine originally founded by Will Eisner). Joe has been illustrating it for years.