This is not precisely known, but it is possible we have been observed for a very long time. Certainly, during and since World War II there has been an increase in sightings and encounters. It appears that these ET visitors are quite concerned with warfare and especially the development and use of nuclear weapons. In a mere 100 years, human technology went from horse and buggies to early space travel and nuclear weapons. This apparently is of some concern to them, for obvious reasons.