We know about the epic drama of World War II, but what about the jokes? The above video tells the story (as best as we can). The iconic piece of graffiti that was known in America as "Kilroy" traveled the world in a fashion remarkably similar to a modern meme.
The drawing — often accompanied by the phrase "Kilroy Was Here" — showed up in unusual places, as if Kilroy himself had scrawled it before anyone else showed up. He was remixed with funny phrases (in homage, a Kilroy accompanied by "Up Yours, Baby" ended the WWII movie Kelly's Heroes), and he showed up everywhere GIs went.
We can't be certain where Kilroy came from — the most popular, near-contemporaneous accounts identify James J. Kilroy as the man who started the tradition. The theory is that Kilroy, a shipyard inspector, wrote "Kilroy Was Here" on his inspected work in order to track it. As the parts traveled the world, GIs paired it with a funny cartoon (possibly inspired by a British and/or Australian doodle, named Mr. Chad and FOO, respectively). However, every town had its own version of the Kilroy myth, so we may never know where it began.
But we do know it made an impact — Kilroy showed up in popular culture, including movies, songs, and more. If you want to delve into more mythology and sightings, you can do so here. But most importantly, he showed up on the unrecorded alley walls, bathroom stall doors, and other hidden places that amused and inspired everyone who saw his face.