Not a lot, since a lot of my research of late has involved The Invaders.
When I first saw The Brotherhood of the Bell (1970) it was my first encounter with the concept of an all-powerful secret conspiracy, so I was pleasantly surprised that it holds up well on rewatching. Glenn Ford is a member of the eponymous society who rebels when its machinations drive a friend to suicide (the weakest part of the film—I kept expecting to learn the brotherhood had murdered him), only to discover that not only has The Bell shaped his entire life (“You haven’t competed for anything in 22 years.”) but is capable of obtaining secret government documents, impersonating federal agents, and triggering tax audits and job blacklists. While not exactly a Fifth Column, the brotherhood is the kind of secret group (real or imagined) that’s provoked American paranoia for years (in one scene, Ford is accused of being a front man for either the Elders of Zion or the Jesuits), so it’s in the book.
The Thing (1982) is one of the thrillers in which personal paranoia and political paranoia are linked—while the alien shapeshifter’s goal is ultimate planetary takeover, the immediate effect is personal as the occupants of an arctic research station try to figure out Are You One Of Us, Or—. Despite the gratuitous special effects and gore, this John Carpenter film is better than I expected.
Invasion USA (1951) isn’t really a Fifth Column film but it’s certainly paranoid, as Russian forces occupy the West Coast, nuke American cities and capture Washington, all because the lead characters didn’t love America enough to be dedicated to the fight against her enemies! This is one of the most authoritarian films I’ve ever seen, implying the only objections to drafting factory workers or forcing manufacturers to switch to munitions are selfish ones—when a tractor manufacturer points out his dealers will lose money if he switches to tanks, an Army officer tells him that Real Americans would be OK with that.
Big Jim McLain (1952) stars John Wayne and James Arness as investigators for the House Un-American Activites Committee, who in this film are not a bunch of witch-hunters but modern American heroes rooting out murderers and traitors (who then go on to hide their evil by invoking the Bill of Rights). One of the most rabid Red-baiting films I’ve seen, equating American Communists to North Korean soldiers and at one point suggesting Communists are more repulsive than lepers. Memorable.