From July 12 to 29, 1952, a series of unidentified flying object (UFO) sightings were reported in Washington, D.C., and later became known as the Washington flap, the Washington National Airport Sightings, or the Invasion of Washington. The most publicized sightings took place on consecutive weekends, July 19–20 and July 26–27. UFO historian Curtis Peebles called the incident "the climax of the 1952 (UFO) flap"—"Never before or after did Project Blue Book and the Air Force undergo such a tidal wave of (UFO) reports."
1952 UFO flap.
The 1952 UFO flap was an unprecedented rash of media attention to unidentified flying object reports during the summer of 1952 that culminated with reports of sightings over Washington, D.C. In the four years prior, the US Air Force had chronicled a total of 615 UFO reports; during the 1952 flap, they received over 717 new reports. Project Blue Book director Edward J. Ruppelt later recalled: "During a six-month period in 1952... 148 of the nation's leading newspapers carried a total of over 16,000 items about flying saucers."
On May 12, an explosion in Seattle led to public speculation of a 'flying saucer'. 
On April 3, the Associated Press reported on an upcoming story in "Life" magazine that would reveal the Air Force was taking a serious interest in flying saucers.
The June edition of "Look" magazine featured a story where astrophysicist Donald Howard Menzel proposed flying saucers were optical mirages created by temperature inversions. American papers covered similar statements from French astronomer Ernest Esclangon who debunked the "flying saucer reports" by explaining they could not be supersonic craft because no sonic booms were reported.
On April 7, "Life" magazine, featuring Marilyn Monroe on its cover, published the Flying Saucer article under the title "Have We Visitors From Space?", becoming the most reputable outlet to seriously consider the possibility that flying saucer reports might be caused by extra-terrestrial spaceships. Publicity surrounding the piece is believed to have contributed to the subsequent wave of reports that summer.
In his book, Ruppelt argued "If you can pin down a date that the Big Flap started, it would probably be about June 1." That day, a UFO report was filed by Hughes Aircraft Company in Los Angeles. On July 1, two F-94s scrambled to intercept a UFO over Boston. On July 14, the Nash-Fortenberry UFO sighting was reported over Chesapeake Bay, when two commercial pilots (William B. Nash and William H. Fortenberry) claimed to have seen eight UFOs flying in a tight echelon formation.
Sightings over Washington, D.C..
At the height of the UFO flap, there were UFOs reported at the nation's capital on two consecutive Saturday nights.
Night of July 19.
At 11:40 p.m. on Saturday, July 19, 1952, Edward Nugent, an air traffic controller at Washington National Airport, spotted seven objects on his radar. The objects were located 15 miles (24 km) south-southwest of the city; no known aircraft were in the area, and the objects were not following any established flight paths. Nugent's superior, Harry Barnes, a senior air-traffic controller at the airport, watched the objects on Nugent's radarscope. He later wrote: "We knew immediately that a very strange situation existed ... their movements were completely radical compared to those of ordinary aircraft."
Barnes had two controllers check Nugent's radar; they found that it was working normally. Barnes then called National Airport's radar-equipped control tower; the controllers there, Howard Cocklin and Joe Zacko, said that they also had unidentified blips on their radar screen, and saw a hovering "bright light" in the sky, which departed with incredible speed. Cocklin asked Zacko, "Did you see that? What the hell was that?"
At this point, other objects appeared in all sectors of the radarscope; when they moved over the White House and the United States Capitol, Barnes called Andrews Air Force Base, located 10 miles from National Airport. Although Andrews reported that they had no unusual objects on their radar, an airman soon called the base's control tower to report the sighting of a strange object. Airman William Brady, who was in the tower, then saw an "object which appeared to be like an orange ball of fire, trailing a tail ... [it was] unlike anything I had ever seen before." As Brady tried to alert the other personnel in the tower, the strange object "took off at an unbelievable speed.
On one of National Airport's runways, S.C. Pierman, a Capital Airlines pilot, was waiting in the cockpit of his DC-4 for permission to take off. After spotting what he believed to be a meteor, he was told that the control tower's radar had detected unknown objects closing in on his position. Pierman observed six objects—"white, tailless, fast-moving lights"—over a 14-minute period. Pierman was in radio contact with Barnes during his sighting, and Barnes later related that "each sighting coincided with a pip we could see near his plane. When he reported that the light streaked off at a high speed, it disappeared on our scope."
Meanwhile, at Andrews Air Force Base, the control tower personnel were tracking on radar what some thought to be unknown objects, but others suspected, and in one instance were able to prove, were simply stars and meteors. However, Staff Sgt. Charles Davenport observed an orange-red light to the south; the light "would appear to stand still, then make an abrupt change in direction and altitude ... this happened several times." At one point both radar centers at National Airport and the radar at Andrews Air Force Base were tracking an object hovering over a radio beacon. The object vanished in all three radar centers at the same time.
At 3 a.m., shortly before two United States Air Force F-94 Starfire jet fighters from New Castle Air Force Base in Delaware arrived over Washington, all of the objects vanished from the radar at National Airport. However, when the jets ran low on fuel and left, the objects returned, which convinced Barnes that "the UFOs were monitoring radio traffic and behaving accordingly". The objects were last detected by radar at 5:30 a.m.
Publicity and Air Force reaction.
The sightings of July 19–20, 1952, made front-page headlines in newspapers around the nation. A typical example was the headline from the "Cedar Rapids Gazette" in Iowa. It read "SAUCERS SWARM OVER CAPITAL" in large black type. By coincidence, USAF Captain Edward J. Ruppelt, the supervisor of the Air Force's Project Blue Book investigation into UFO sightings, was in Washington at the time. However, he did not learn about the sightings until Monday, July 21, when he read the headlines in a Washington-area newspaper. After talking with intelligence officers at the Pentagon about the sightings, Ruppelt spent several hours trying to obtain a staff car so he could travel around Washington to investigate the sightings, but was refused as only generals and senior colonels could use staff cars. He was told that he could rent a taxicab with his own money; by this point Ruppelt was so frustrated that he left Washington and flew back to Blue Book's headquarters at Wright-Patterson AFB in Dayton, Ohio. Upon returning to Dayton, Ruppelt spoke with an Air Force radar specialist, Captain Roy James, who felt that unusual weather conditions could have caused the unknown radar targets.
On July 24, two Air Force colonels took off from Hamilton Air Force Base bound for Colorado Springs; they reported witnessing unidentified triangular objects. 
Night of July 26.
At 8:15 p.m. on Saturday, July 26, 1952, a pilot and stewardess on a National Airlines flight into Washington observed some lights above their plane. Within minutes, both radar centers at National Airport, and the radar at Andrews AFB, were tracking more unknown objects. USAF master sergeant Charles E. Cummings visually observed the objects at Andrews, he later said that "these lights did not have the characteristics of shooting stars. There was [sic] no trails ... they traveled faster than any shooting star I have ever seen."
Meanwhile, Albert M. Chop, the press spokesman for Project Blue Book, arrived at National Airport and, due to security concerns, denied several reporters' requests to photograph the radar screens. He then joined the radar center personnel. By this time (9:30 p.m.) the radar center was detecting unknown objects in every sector. At times the objects traveled slowly; at other times they reversed direction and moved across the radarscope at speeds calculated at up to 7,000 mph (11,250 km/h). At 11:30 p.m., two U.S. Air Force F-94 Starfire jet fighters from New Castle Air Force Base in Delaware arrived over Washington. Captain John McHugo, the flight leader, was vectored towards the radar blips but saw nothing, despite repeated attempts. However, his wingman, Lieutenant William Patterson, did see four white "glows" and chased them. He told investigators that "I tried to make contact with the bogies below 1,000 feet," and that "I was at my maximum speed but...I ceased chasing them because I saw no chance of overtaking them." According to Albert Chop, when ground control asked Patterson "if he saw anything", Patterson replied "'I see them now and they're all around me. What should I do?'...And nobody answered, because we didn't know what to tell him."
After midnight on July 27, USAF Major Dewey Fournet, Project Blue Book's liaison at the Pentagon, and Lt. John Holcomb, a United States Navy radar specialist, arrived at the radar center at National Airport. During the night, Lieutenant Holcomb received a call from the Washington National Weather Station. They told him that a slight temperature inversion was present over the city, but Holcomb felt that the inversion was not "nearly strong enough to explain the 'good and solid' returns" on the radar scopes. Fournet relayed that all those present in the radar room were convinced that the targets were most likely caused by solid metallic objects. There had been weather targets on the scope too, he said, but this was a common occurrence and the controllers "were paying no attention to them". Two more F-94s from New Castle Air Force Base were scrambled during the night. One pilot saw nothing unusual; the other pilot saw a white light which "vanished" when he moved towards it. Civilian aircraft also reported glowing objects that corresponded to radar blips seen by Andrews radar operators. As on July 20, the sightings and unknown radar returns ended at sunrise.
Aftermath.
On August 19, West Palm Beach scoutmaster Sonny DesVergers reported being burned by a flying disc.
Air Force explanation.
Air Force Major Generals John Samford, USAF Director of Intelligence, and Roger M. Ramey, USAF Director of Operations, held a well-attended press conference at the Pentagon on July 29, 1952. In his opening comments, he noted that, out of the hundreds of UFO reports in recent years investigated by the Air Force, there was "a certain percentage of this volume of reports that have been made by credible observers of relatively incredible things" but that none of them posed any national security threat. At the event, Samford stated that the visual sightings over Washington could be explained as misidentified aerial phenomena such as stars or meteors, and unknown radar targets could be explained by temperature inversion, which was present in the air over Washington on both nights the radar returns were reported. In addition, Samford stated that the unknown radar contacts were not caused by solid material objects, and therefore posed no threat to national security. In response to a question as to whether the Air Force had recorded similar UFO radar contacts prior to the Washington incident, Samford said that there had been "hundreds" of such contacts where Air Force fighter interceptions had taken place, but stated that they were all "fruitless". It was the largest Pentagon press conference since World War II. Press stories called Samford and Ramey the Air Force's two top UFO experts.
Among the witnesses who supported Samford's explanation was the crew of a B-25 bomber, which had been flying over Washington during the sightings of July 26–27. The bomber was vectored several times by National Airport over unknown targets on the airport's radarscopes, yet the crew could see nothing unusual. Finally, as a crew member related, "the radar had a target which turned out to be the Wilson Lines steamboat trip to Mount Vernon... the radar was sure as hell picking up the steamboat." Air Force Captain Harold May was in the radar center at Andrews AFB during the sightings of July 19–20. Upon hearing that National Airport's radar had picked up an