Somehow, Kilroy's message combined with the already popular image of Mr. Chad became so popular that GIs began to scribble Kilroy's name and new identity onto everything they passed throughout the war: bunkers, bridges, walls, the Arc de Triomphe and likely Hitler's bathroom at Eagle's Nest. Once the war was won, an epic race broke out to stick Kilroy's name and face on whatever else existed anywhere, from the Statue of Liberty to the Berlin Wall to, no joke, the moon. At the height of the fad, it was common for pranksters to approach some landmark or another and whip out their markers, only to find that Kilroy had already been there.
Via Florian Kilzer
Needless to say, this was probably the last pop culture phenomenon to emerge from the profession of industrial riveting.