General Douglas MacArthur was an American war hero and one of the most popular armed service members of the 1940s and 50s. But what few know about him is his strong interest in the UFO phenomenon, so much so that he warned of “sinister forces” beyond Earth three different times publicly in the last 15 years of his life.
Sources in order of mention
1951 speech https://archive.org/details/ProjectBlueBookGuide/17.mp3
1962 speech http://penelope.uchicago.edu/Thayer/E/Gazetteer/Places/America/United_States/Army/USMA/MacArthur/1962_speech_to_the_Corps.html
Quote in Space Force newsletter https://www.afspc.af.mil/Portals/3/documents/HF/AFD-070622-058.pdf
John Keel article on MacArthur collecting UFO reports and a 1958 briefing where he warned others his interpretation was it was a “scouting” force: https://www.scribd.com/document/230363655/General-Douglas-MacArthur-Ufologist-by-John-A-Keel
MacArthur parade in Chicago April 26,1951, Aprad (Creative Commons Attribution-Share Alike 4.0 International): https://commons.wikimedia.org/wiki/File:MacArthur_parade_in_Chicago_April_26,1951.jpg