According to George Lenczowski, as early as May 23, President Johnson secretly authorized supplying Israel by air with a variety of arms systems, even when an embargo on weapons shipments was placed on the Middle East.

Stephen Green wrote in his book that the United States sent reconnaissance aircraft to track nighttime movement of Egyptian ground forces in order to facilitate daytime Israeli air attacks that proved important for Israel's advances. Richard Parker disputes this and suggests that it is a hoax, based on the questionable testimony of a single man.

On the second day of the war, Arab state-run media reported that American and British troops were fighting on Israel's side. Radio Cairo and the government newspaper Al-Ahram made a number of claims, among them: that U.S. and British carrier-based aircraft flew sorties against the Egyptians; that U.S. aircraft based in Wheelus Air Base-Libya attacked Egypt; and that American spy satellites provided imagery to Israel. Mohamed Hassanein Heikal, the chief of “Al-Ahram” in the Nasserite period, repeated similar claims at Al Jazeera channel. Later, Muammar al-Gaddafi's Libyan government confirmed these claims also only to get a pretext for the coup that took place on 1 September 1969. The governments of United States and Britain made little effort to either confirm or deny these claims. Similar reports were aired by Radio Damascus and Radio Amman. Egyptian media even said that King Hussein had personally seen radar observations showing British aircraft taking off from aircraft carriers.

Outside of the Arab world, claims of American and British military intervention were not taken seriously. Britain, the U.S. and Israel denied these allegations. On June 8, Egyptian credibility was further damaged when Israel released an audio recording to the press, which they said was a radio-telephone conversation intercepted two days earlier between Nasser and King Hussein of Jordan.

Nasser: ...Shall we include also the United States? Do you know of this, shall we announce that the U.S. is cooperating with Israel?

Hussein: Hello. I do not hear, the connection is the worst - the line between you and the palace of the King from which the King is speaking is bad.
Nasser: Hello, will we say the U.S. and England or just the U.S.?
Hussein: The U.S. and England.
Nasser: Does Britain have aircraft carriers?
Hussein: (Answer unintelligible).
Nasser: Good. King Hussein will make an announcement and I will make an announcement. Thank you... Will his Majesty make an announcement on the participation of Americans and the British?
Hussein: (Answer unintelligible).
Nasser: By God, I say that I will make an announcement and you will make an announcement and we will see to it that the Syrians will make an announcement that American and British airplanes are taking part against us from aircraft carriers. We will issue an announcement, we will stress the matter and we will drive the point home.

In the immediate aftermath of the war, as the extent of the Arab military defeat became apparent, Arab leaders differed on whether to continue to assert that the American military had assisted the Israeli victory. On June 9, 1967, Nasser stated in his resignation speech (his resignation was not accepted):

What is now established is that American and British aircraft carriers were off the shores of the enemy helping his war effort. Also, British aircraft raided, in broad daylight, positions of the Syrian and Egyptian fronts, in addition to operations by a number of American aircraft reconnoitering some of our positions... Indeed, it can be said without exaggeration that the enemy was operating with an air force three times stronger than his normal force.

King Hussein, however, later denied the allegations of American military support. On June 30, he announced in New York that he was "perfectly satisfied" that "no American planes took part, or any British planes either". In September, The New York Times reported that Nasser had privately assured Arab leaders, gathered in Sudan to discuss the Khartoum Resolution, that his earlier claims were false.

Nonetheless, these allegations, that the Arabs were fighting the Americans and British rather than Israel alone, took hold in the Arab world. As reported by the British Representative in Jeddah, Saudi Arabia, a country at odds with Egypt as a result of the Yemen war:

President Abdel Nasser's allegation ... is firmly believed by almost the whole Arab population here who listen to the radio or read the press ... Our broadcast denials are little heard and just not believed. The denials we have issued to the broadcasting service and press have not been published. Even highly educated persons basically friendly to us seem convinced that the allegations are true. Senior foreign ministry officials who received my formal written and oral denials profess to believe them but nevertheless appear skeptical. I consider that this allegation has seriously damaged our reputation in the Arab world more than anything else and has caused a wave of suspicion or feeling against us which will persist in some underlying form for the foreseeable future ... Further denials or attempts at local publicity by us will not dispel this belief and may now only exacerbate local feeling since the Arabs are understandably sensitive to their defeat with a sense of humiliation and resent self-justification by us who in their eyes helped their enemy to bring this about.

Well after the end of the war, the Egyptian government and its newspapers continued to make claims of collusion between Israel, the United Kingdom and the United States. These included a series of weekly articles in Al-Ahram, simultaneously broadcast on Radio Cairo by Mohamed Heikal. Heikal attempted to uncover the "secrets" of the war. He presented a blend of facts, documents, and interpretations. Heikal's conclusion was clear-cut: there was a secret U.S.-Israeli collusion against Syria and Egypt.

According to Israeli historian Elie Podeh: "All post-1967 [Egyptian] history textbooks repeated the claim that Israel launched the war with the support of Britain and the United States. The narrative also established a direct link between the 1967 war and former imperialist attempts to control the Arab world, thus portraying Israel as an imperialist stooge. The repetition of this fabricated story, with only minor variations, in all history school textbooks means that all Egyptian schoolchildren have been exposed to, and indoctrinated with, the collusion story." The following example comes from the textbook Al-Wisam fi at-Ta'rikh:

The United States' role: Israel was not (fighting) on its own in the (1967) war. Hundreds of volunteers, pilots, and military officers with American scientific spying equipment of the most advanced type photographed the Egyptian posts for it (Israel), jammed the Egyptian defense equipment, and transmitted to it the orders of the Egyptian command.

In Six Days of War, historian Michael Oren argues that the Arab leadership spread false claims about American involvement in order to secure Soviet support for the Arab side. After the war, as the extent of the Israeli victory became apparent to the Arab public, these claims helped deflect blame for the defeat away from Nasser and other Arab leaders. In reaction to these claims, Arab oil-producing countries announced either an oil embargo on the United States and Britain or suspended oil exports altogether.

Six Arab countries broke off diplomatic relations with the United States, and Lebanon withdrew its Ambassador. More broadly, the Six Day war hastened the process of anti-American radicalization in the Middle East, a process expressed by the growth of both leftist and religious-fundamentalist movements and their increased resort to terrorism as a weapon in their anti-American struggle. In fact, it transcended the Arab countries and spread to Iran, Pakistan and the Third World, whose delegates at the UN began adopting increasingly critical posture toward America.

A British guidance telegram to Middle East posts concluded: "The Arabs' reluctance to disbelieve all versions of the big lie springs in part from a need to believe that the Israelis could not have defeated them so thoroughly without outside assistance."