Was the UN critically involved in defusing the Cuban Missile Crisis?

First of all, seeing as to how more than one SE user has questioned the seriousness of the Cuban missile crisis, let me try to outline how tense things were at the time.

1. The Cuban missile crisis is the only time ever that any section of the US military has mobilised to DEFCON 2. The erstwhile SAC was at DEFCON 2 while the rest of the armed forces were at DEFCON 3.

2. Kennedy had informed his military advisers (who were champing at the bit to invade Cuba and depose Castro) that if any of the recon flights were attacked, that he would authorise (a planned) invasion of Cuba. Kennedy eventually preferred to "quarantine" the island instead even after a U-2 was shot down. Also,

   Kennedy had explicitly promised the American people less than a month before the crisis that "if Cuba should possess a capacity to carry out offensive actions against the United States...the United States would act."

3. The Soviet commanders in control of the nukes on Cuba were authorised to launch if the US ever invaded Cuba. Castro himself advocated their use if the invasion happened and as the crisis deepened, tacitly—in a letter to the Soviets—encouraged a first strike against the "perfidious imperialists".

   At the height of the missile crisis, on Oct. 27, when the world seemed poised on the edge of nuclear holocaust, Castro had appeared to urge Moscow to launch a first-strike nuclear attack on America.

   "If the imperialists invade Cuba," Castro wrote in a letter to Khrushchev, "the danger that that aggressive policy poses for humanity is so great that following that event, the Soviet Union must never allow the circumstances in which the imperialists could launch the first nuclear strike.

   "If they actually carry out the brutal act of invading Cuba ... that would be the moment to eliminate such danger forever through an act of legitimate self-defense, however harsh and terrible the solution would be."

So, it's safe to conclude that this was a brink-of-war crisis.

There were at least two reasons for the Soviets to want to arm Cuba:

• Cuba wanted defensive capabilities and, as was later revealed, offensive capabilities as well.
• The USSR were not too taken with the presence of Jupiter MRBMs in Turkey and Italy.

This was also not long after the failed Bay of Pigs invasion which was an abject failure for the Kennedy government and provided Castro with a lot of political currency and sympathy.

When the crisis broke in mid-October, the American ambassador to the UN, Adlai Stevenson argued for a diplomatic compromise with the USSR and Cuba by:

• withdrawing the Jupiter missiles from Turkey and Italy.
• demilitarising Guantanamo Bay.

Nobody really openly supported his idea and many of the members of the executive committee were openly against it preferring to take a harder line which included a naval quarantine and a possible invasion of Cuba.

When Stevenson put forth his plan for the missile trade to the ExComm, including the evacuation of Guantánamo, he did not reject the possibility that U.S. military action might still be required. Stating his position in a letter to the president, Stevenson emphasized that "the national security must come first." However, the military "means adopted have such incalculable consequences that I feel you should have made it clear that the existence of nuclear missile bases anywhere is NEGOTIABLE before we start anything ..." (italics in original). Despite his hope for a diplomatic solution, Stevenson also acknowledged that "we can't negotiate with a gun at our head" and "if they won't remove the missiles [from Cuba] and restore the status quo ante we will have to do it ourselves."

According to Stevenson's biographer, the ambassador defended his approach to the situation to Kenneth O'Donnell at a party during the crisis: "I know that most of those fellows will probably consider me a coward for the rest of my life for what I said today. But perhaps we need a coward in the room when we are talking about nuclear war."

The preference for a political settlement rather than an air-strike and an invasion of Cuba was due to the fact that the US, at the time, did not have any concrete proof that the missiles in Cuba were anything but defensive. Cuba had plenty of reasons to defend herself from her capitalistic neighbour and in the UN, the Soviets stubbornly stuck to the position that they were simply helping their communist brethren defend themselves.

With Kennedy hesitant to launch any sneak attacks on Cuba, Stevenson was asked to exert pressure on the Soviets via the UN.

Robert Kennedy nevertheless sent Arthur Schlesinger to the UN with Stevenson, instructing him, "We're counting on you to watch things in New York. That fellow is ready to give everything away."

Stevenson did everything but that. On the 25th of October, armed with new evidence, he set out to trap his Soviet counterpart in the UN, Vladimir Zorin.

That afternoon at an emergency meeting of the Security Council, Ambassador Zorin assured his fellow delegates that the Soviet Union had not placed missiles in Cuba: “Falsity is what the United States has in its hands, false evidence.” The United States, he argued, was manufacturing a threat that could have “catastrophic consequences for the whole world.”

Stevenson listened impassively as the Soviet ambassador laced into the United States. When it was finally his turn to speak, he dispensed with the standard diplomatic niceties. He instead went immediately for the jugular: “I want to say to you, Mr. Zorin, that I do not have your talent for obfuscation, for distortion, for confusing language, and for doubletalk. And I must confess to you that I am glad that I do not!” Stevenson went on to denounce the Soviets for lying, treating Zorin in a way that the Soviet ambassador likened to an American prosecutor browbeating a defendant. Stevenson pressed on:

All right, sir, let me ask you one simple question: Do you, Ambassador Zorin, deny that the U.S.S.R. has placed and is placing medium- and intermediate-range missiles and sites in Cuba? Yes or no—don’t wait for the translation—yes or no?

When Zorin refused to answer, Stevenson snapped:

You can answer yes or no. You have denied they exist. I want to know if I understood you correctly. I am prepared to wait for my answer until hell freezes over, if that’s your decision. And I am also prepared to present the evidence in this room

With Zorin still continuing to refuse to answer, Stevenson’s aides proceeded to produce large photos of the Soviet missiles in Cuba. The delegates in the room, and everyone watching on television, saw the Soviets unmasked as liars. Zorin could only simmer. The mild-mannered Stevenson had scored an enormous political and diplomatic victory for the United States.

The rather exaggerated last paragraph notwithstanding, this exchange tilted world opinion in favour of the United States.

In the meantime, the Secretary General of the UN, U Thant, who had been functioning as a mediator, pressed Premier Khrushchev to negotiate.

The newspapers of the day recognized and lauded Thant for his contribution, but US historians later glossed over his role. Nevertheless, State Department and UN archival documents as well as presidential recordings, show that Thant's mediation was vital in helping Kennedy and Khrushchev move away from nuclear Armageddon. Indeed, after the crisis, the president said: "U Thant has put the world deeply in his debt."

That said, it was on the 27th of October, two days later, that the worst of the crisis came to pass as a U-2 plane was shot down over Cuba. But eventually, cooler heads prevailed and negotiations continued with an agreement finally reached to dismantle and withdraw missiles from Cuba, Turkey and Italy which was essentially the same plan suggested by Stevenson to JFK and his team.

While I am not suggesting that Stevenson and Thant were the only people responsible for solving the crisis, it is my considered opinion—based on the above evidence—that they each, as UN officials, played critical roles in defusing it.

From The Guardian:

The "Adlai Stevenson moment" has become shorthand in Washington for the elusive conclusive proof that Iraq is concealing weapons of mass destruction.

Recommended reading: Thirteen Days: A Memoir of the Cuban Missile Crisis by Robert F. Kennedy (1969)