Though all the various plans for invasion of the Earth by aliens are convoluted and intertwined and thus hard to separate, there is clearly one that stands out as the first since it began millions of years ago.
The Beginning
The aliens first came to earth in the form of a virus carried along by a meteor crashing to earth millions of years in the past. After lying dormant for some time while life on earth began, the virus eventually became active taking over one of the branches of early man and thus altering human genetics for all time. Several aliens were spawned during this time (at least to stage 2) but would eventually die out. The virus, however, would remain dormant in deposits of black oil throughout the globe for years to come.
The Return
Despite lying dormant, the viral organism’s ability to communicate with members of its own race would remain. Throughout the next several millenia, many alien visitors would be drawn to the earth by this communication such as those that visited the Anasazi in the New Mexico desert and presumably the Mayan civilization. Many of these ancient cultures already saw a threat in these arrivals and found ways to combat them but these were lost to time. Along the way, aliens may have been influencing the direction of mankind as a whole since alien artifacts have been found with writings that translate to ancient religious texts.
Roswell
As man had entered the modern era, plans soon began to be made by the aliens to reclaim the planet in secret. The unexpected happened, however, when an alien craft crashed (or was shot down) in Roswell, New Mexico. Certain military and government officials became of aware of this event, and the analysis of remains from the crash (and discovery of a living alien?) finally made mankind aware of the aliens, their technology, and their plans for colonization.