The post World War II UFO phase in the United States began with a reported sighting by American businessman Kenneth Arnold on June 24, 1947 while flying his private plane near Mount Rainier, Washington. He reported seeing nine brilliantly bright objects flying across the face of Rainier towards nearby Mount Adams at "an incredible speed", which he "calculated" as at least 1200 miles per hour by timing their travel between Rainier and Adams.

His sighting subsequently received significant media and public attention. Arnold would later describe what he saw as being "flat like a pie pan" and as flying "like a saucer would if you skipped it across the water" and also said they were and "half-moon shaped, oval in front and convex in the rear. ...they looked like a big flat disk." (One, however, he would describe later as being almost crescent-shaped.) Arnold’s reported descriptions was widely reported upon and gave rise to the terms flying saucer and flying disk.^[8] Arnold’s sighting was followed in the next few weeks by hundreds of other reported sightings, mostly in the U.S., but in other countries as well.

After reports of the Arnold sighting hit the media, other cases began to be reported in increasing numbers. In one instance a United Airlines crew sighting of nine more disc-like objects over IdahoJuly 4. At the time, this sighting was even more widely reported than Arnold’s and lent considerable credence to Arnold’s report. on the evening of

American UFO researcher Ted Bloecher, in his comprehensive review of newspaper reports, found a sudden surge upwards in sightings on July 4, peaking on July 6–8. Bloecher noted that for the next few days most American newspapers were filled with front-page stories of the new "flying saucers" or "flying discs". Reports began to tail off after July 8, when officials began issuing press statements on the Roswell UFO incident,^{[citation needed]} in which they explained the debris as being that of a weather balloon.

Over several years in the 1960s, Bloecher (aided by physicist James E. McDonald) discovered 853 flying disc sightings that year from 140 newspapers from Canada, Washington D.C, and every U.S. state except Montana.