While similar events to the classic abduction have been reported though out history where the aliens are called ether hags or demons, interest in UFO's started in the late 1940's after we started using nuclear bombs.
There was a number of sightings by air force and of course the Roswell incident, where ether a spaceship or a balloon crashed. whatever happened something was being covered up by the U S government. The official explanation is that it was a military balloon used in "project mogul.
In the 1950 and 1960 the reports of aliens claim that they friendly and they warn of the dangers of using nuclear power.
In the 1940's the U S government created project Blue book to report UFO incidents it was supposably closed in 1969 but recent reports suggest it was reopened in 1989.
In 1952 the US government started a secret project called Majestic12.
The first reported alien abduction was that of Betty & Barney Hill in 1961 who were abducted while they were in their car the car was found to be magnetized. After hypnosis it was found that they come from a star system called "Zeta reticulum.
Another important abduction was that of Travis Walton in 1975 . where the people involved passed the polygraph test ( lie detector ). This case was used in the film "Fire in the Sky".
During the 1980's and 1990 alien encounters became more sinister with increase in the number of reported abductions, where the victims were experimented on to create hybrids.
This increase in UFO activity fueled rumors of a invasion in the year 2000 along with other apocalyptic predictions for the millennium.