The public rarely hears about interactions between military personnel and unexplained aircraft -- especially during wartime.
As time goes on, however, UFO stories stuck behind red tape begin to see the light of day. The Vietnam War saw its share of UFO activity in the 1960s.
One close encounter, in 1968, involved the crew of an American patrol boat that reported two glowing circular craft following them in the demilitarized zone that separated North and South Vietnam (see depiction above).
The crew aboard a second patrol boat later reported seeing the UFOs over the first boat and a flash of light, followed by an explosion that completely destroyed the boat. These Vietnam reports included close observation of the unknown aerial craft which appeared to house pilots (see recreation image at the right).
Wartime UFO stories are recreated in the premiere episode of the second season of History's "Hangar 1: The UFO Files." The accounts are drawn from tens of thousands of UFO cases in the archives of the Mutual UFO Network, the world's largest UFO investigation group.
"The military was interested in [UFOs] because they had capabilities far above anything that we had, and they wanted to find out what the technology was and, frankly, who they belonged to," according to former Air Force intelligence officer, Capt. George Filer.
While in Vietnam, Filer -- who had a top secret clearance -- gave daily briefings to Gen. George S. Brown, deputy commander for air operations in Vietnam.
"Frequently, the Vietcong or North Vietnamese would be attacking an outpost and I would explain that, and we would have ground-air support, particularly at night where we'd go in there with these gun ships, and I would give briefings on all of that," Filer told The Huffington Post. "Some of the time, there would be unidentified craft over the DMZ."
Filer described a typical report that he'd receive and which he included in his briefings to Brown:
"You'd have an aircraft flying along, doing around 500 knots and a UFO comes alongside and does some barrel rolls around the aircraft and then flies off at three times the speed of one of the fastest jets we have in the Air Force. So, obviously, it has a technology far in advance of anything we have."
"I would be told this unofficially. People tell you a lot of things that they don't put in writing or sign their name to. There was always this part of UFOs that, if you got too interested, it could mess up your career. And this is true today even with commercial pilots. I've also heard from people serving in Afghanistan saying they've seen UFOs, and the Iranian news carries UFO reports pretty regularly."
During a 1973 press conference, five years after the patrol boat UFO encounters, Brown -- as USAF chief of staff -- was asked about the Air Force's position on UFOs:
I don't know whether this story has ever been told or not. They weren't called UFOs. The were called enemy helicopters. And they were only seen at night and they were only seen in certain places. They were seen up around the DMZ in the early summer of '68. And this resulted in quite a little battle.
And in the course of this, an Australian destroyer took a hit and we never found any enemy, we only found ourselves when this had all been sorted out. And this caused some shooting there, and there was no enemy at all involved, but we always reacted.
Always after dark, the same thing happened up at Pleiku at the Highlands in '69.
Many stories about battling UFOs have emerged throughout history.
Related content: Ancient spaceship battles and UFO sightings in our skies
Filer -- who documents his sightings and other UFO news at the National UFO Center site -- was one of several military eyewitnesses to something extraordinary in the sky over England. It was 1962, and he was the navigator on a refueling tanker.
"We were out over the North Sea when London Control called and asked if we would be willing to intercept an unidentified that was over Oxford and the Stonehenge area. We had just finished up our refueling mission, so we said sure, and they cleared all the traffic around us and gave us top priority as we descended towards the UFO. All they really had was a very large radar return, but it was much bigger than a normal aircraft."
Filer (pictured at right) recalled how his radar scope indicated the UFO was as big as the huge Firth of Forth Bridge in Scotland that he and his crew often used as a regular navigation point.
"The 'thing' was at 1,000 feet and we were descending from 32,000 feet. We picked up this huge radar return while we were still about 30 miles out. It was dark out and when we got much closer to the object, we saw lights around it, outlining the shape of a cylinder, like a cruise ship. It then just quickly rose and went up into space.
"We were pretty sure we'd just seen a UFO."
Filer also told HuffPost that he has heard from air traffic controllers who claimed they were told to "always divert aircraft away from UFOs and deny that it existed. I think they want this whole situation to go away, and I think [the policy] is coming from the National Security Council -- they're at the highest level. It sounds funny, but presidents don't always know what their National Security Council is doing."