While unusual sightings have been reported in the sky throughout history, UFOs became culturally prominent after World War II, escalating during the Space Age.
In the Pacific and European theatres during World War II, round, glowing fireballs known as “foo fighters” were reported by Allied and Axis pilots. Some explanations for these sightings included St. Elmo’s fire, the planet Venus, hallucinations from oxygen deprivation, and German secret weapons (specifically rockets).[15] In 1946, more than 2,000 reports were collected, primarily by the Swedish military, of unidentified aerial objects over the Scandinavian nations, along with isolated reports from France, Portugal, Spain, Italy, and Greece. The objects were referred to as “Russian hail” (and later as “ghost rockets”) because it was thought the mysterious objects were possibly Russian tests of captured German V1 or V2 rockets, but most were identified as natural phenomena as meteors.[16]
Many scholars, especially those arguing for the psychosocial UFO hypothesis, have noted that UFO characteristics reported after the first widely publicized modern sighting by Kenneth Arnold in 1947 resembled a host of science fiction tropes from earlier in the century.[17][18][19][20][21][22][23]
The 1947 flying disc craze was a rash of unidentified flying object reports in the United States that were publicized during the summer of 1947. The craze began on June 24, when media nationwide reported civilian pilot Kenneth Arnold’s story of witnessing disc-shaped objects which headline writers dubbed “Flying Saucers”. Such reports quickly spread throughout the United States; historians would later chronicle at least 800 “copycat” reports in subsequent weeks, while other sources estimate the reports may have numbered in the thousands.
They aren’t strictly, speaking.
The UFO craze kicked off just after WW2 in the US, so you initially had a lot there.
https://en.wikipedia.org/wiki/Unidentified_flying_object
https://en.wikipedia.org/wiki/1947_flying_disc_craze