From 1903 to 1942, the American forces used Lily Hill primarily as an observation point and the Japanese followed suit when they occupied Clark. During the American attack on Clark, tunnels were dug beneath its surface and it was from this point that the Japanese made their final defense. After World War II, the USAF established it first aircraft warning and control unit which remained on its summit, until relocated to Wallace Air Station at Poro Point on the Lingayen Gulf coast in December 1962. In 1955, the underbrush on Lily Hill was burned back, revealing the remains of two Japanese aircraft and many smaller artifacts attesting to Japan's strong defense of Clark.