Harry Truman was president of the United States when Kenneth Arnold saw flying saucers over the state of Washington, ushering in the modern era of UFO sightings.
He was president when the Roswell Incident occurred in 1947. He was president in 1952 when UFOs buzzed Washington, D.C. over consecutive days.
Harry Truman was familiar with UFOs—obviously.
It was the UFO fly-bys over D.C. that forced Truman to act. There were unknown, saucer-shaped craft seen zooming over the White House, the Pentagon, and the Capitol building, and newspaper headlines shouted about the sightings. The public was talking about them; in some newspapers, the front-page UFO stories pushed articles about the Democratic National Convention to page two.
Truman’s response? Out of frustration, he gave the order: “Shoot them down!” Several well-known scientists, including Albert Einstein, heard about the order and urged him to rescind it. Truman did, around the time that the UFO sightings also ceased.