In World War 2, part of the park became Camp Griffiss, the base for the US 8th Army Air Force, US Strategic Air Forces and later the Supreme Headquarters Allied Expeditionary Forces. This was where General Eisenhower planned Operation Overlord, the code name for the allied invasion of north-west Europe that began with the D-D landings. Most of the camp's huts were removed by 1963 but you can still see remains of drain covers and stones poking through the grass. There are formal memorials to the USAAF and SHAEF in the north-east corner of the park.
Links: en.wikipedia.org, en.wikipedia.org