[Ozz]

There was one person who led or participated in every combat, training or occupation operation during WWII and the Korean War. This person could always be depended on. GI's began to consider him the "super GI." He was one who always got there first or who was always there when they left. I am, of course, referring to Kilroy Was Here. Somehow, this simple graffiti captured the imagination of GI's everywhere they went. The scribbled cartoon face and words showed up everywhere - worldwide. Stories (some even true) abound.
This Legend of how "Kilroy was here" starts is with James J. Kilroy, a shipyard inspector during WWII. He chalked the words on bulkheads to show that he had been there and inspected the riveting in the newly constructed ship. To the troops in those ships, however, it was a complete mystery — all they knew for sure was that he had "been there first." As a joke, they began placing the graffiti wherever they (the US forces) landed or went, claiming it was already there when they arrived.
Kilroy became the US super-GI who always got there first — wherever GI's went. It became a challenge to place the logo in the most unlikely places. It was said to be atop Mt. Everest, the Statue of Liberty, the underside of the Arch de Triumphe, and scrawled in the dust on the moon. An outhouse was built for the exclusive use of Truman, Stalin, and Churchill who were there for the Potsdam conference. The first person to use it was Stalin. He emerged and asked his aide (in Russian), "Who is Kilroy?"
(Wikipedia)