There are many LEGENDS concerning where this statement originated, as you may view from the link here.
HOWEVER, during WWII it started, it seemed a valuable tool in our lives to bring a smile on one's face in recognition. Almost every VMail received from "over there" from a servicman had the slogan "kilroy was here". It indicated to us that he was in good spirits, and tolerating the gruesome war with a sense of humor, which we had little of during those times.
Our love letters sent to our boy friends would sometimes include this statement, as well as a lipstick print of a kiss.
This statement continued during the WWII years, and long after, as best recalled.
Is understood that servicemen also scribbled on objects in view sometimes, seemed like a nice comforting thought that "Kilroy Was Here". And always placed a smile.
Seemed to be a valuable asset between one another, whether in the states or elsewhere. The saying somehow kept us together, and was comforting reassurance of better days.
KILROY kept us together during WWII.