I recall in the mid 1970s watching repeats of this series just before midnight in a small room surrounded by eight other people, with the lights off, and the pop corn at the ready. Spooky, spooky. The series is my second favourite Sci-Fi series after Star Trek. It was cleverly done, believable, not falling for the Whizz Bang style of today but letting the quite varied story telling and wide open spaces draw you in, with the sinister humourless men in black style and only the occasional alien implement and weapon, that in fact added to their impact, to give you that monster in the closet feel. I've seen no other alien film or TV series other than Alien, to give that constant sense of foreboding and chill. It was a masterpiece of paranoia, and well suited to the Cold War mentality of the time. And yep, the aliens were obviously meant to represent some nefarious activities of a totalitarian regime undermining the clean living American citizens (?), maybe Nazi, maybe Russian, maybe Chinese.
The imagery was very average country homeland USA, ordinary people, few suspecting what was happening next door, government officials slow to pick up the hints, that an underground of terror was on the move. Roy Thinnes was ideal for the role, a Mr Average, vulnerable but determined to do his best to stop those nasty aliens and their corrupting influences. This is a classic that sadly only lasted two seasons. A poor quality movie was made years later which I would suggest avoiding. But get the series because I feel it was one of the best Sci-Fi series ever made.