It's also revealed their species is combating another who's even worse. This conflict boils over to the present in the novels. Basically all myths of gods, ancient heroes and lost lands (like Atlantis) come from their past interference. In fact two factions exist who have both battled each other for millennia, and their leaders took various guises (such as King Arthur) to win humans' support. They were the creators of most famous edifices (the Pyramids, the Great Wall of China, Stonehenge etc.).

Ancient Astronauts: The series reveals there is an alien species that has interfered with humanity throughout our history, and has maintained a presence into the modern day.In fact, the author himself claims 95% of the content is true.

were all or mostly true according to the books.

All Theories Are True: The prevalent occult/conspiracy theories about ancient aliens on Earth, Atlantis, Area 51 the Pyramids etc.

Aliens Steal Cattle: Part of the Area 51 disinformation campaign is taking cattle and mutilating them with lasers, which makes people think aliens did it for some unknown reason, misdirecting them down that rabbit hole.

The Swarm, meanwhile, view all other life forms as hosts.

Aliens Are Bastards: The Airlia are coldly indifferent to humanity at best, and see us only as minions.

Once their past presence has been made known, they reveal themselves fully. This turns out to be wrong-some never left, and have been manipulating humanity from the shadows.

Aliens Among Us: Initially humans in the know think the aliens whose technology they've gotten hold of left millennia ago.

People abducted are implanted with fake memories which make it appear real.

Alien Abduction: These were actually faked by Majic-12 to keep people in the dark about what's really going on.

The Ageless: As a result of their technology, Airlia can keep from aging (along with humans they've enhanced) and don't die of this.

However, they were fighting something even worse.

Abusive Precursors: The Airlia, the aliens who created humans and have covertly manipulated us throughout history, view humanity as cannon fodder at best.

The series contains examples of the following tropes: Thus far, there have been thirteen books in the series, along with some spin-offs which involve elements from the books' past. It begins revolving around the titular secret military site in Nevada, though rapidly expands to a global and then interstellar scale with the reveal of what's going on there, plus its connections to other things around the world. The cover of the first novel's e-book edition Area 51 is a long-running book series by Bob Mayer, starting under his pen name of Robert Doherty in 1997 and continuing to this day.

Area 51 by Robert Doherty