The non-invasion pledge was not a formal goal of the Soviets, but rather something that covered two important diplomatic bases for the Soviets.

One, the pledge was a face-saving measure for the Soviets to make good their removal of the missiles. The American tit-for-tat withdrawal of their Jupiter missiles in Turkey was kept secret as part of the US-Soviet agreement to resolve the crisis. To withdraw the missiles without gaining anything public in return would have been an embarrassment for Soviet diplomacy and power. This was no laughing matter given the Sino-Soviet split and Mao's constant hectoring of the Soviets as old men whose commitment to world revolution was all bark with no bite. Beijing directed a public diplomacy effort that stressed the West was a "paper tiger" and that Soviets did not have the stomach to confront the West on the behalf of the Third World. Forestalling Chinese inroads into Cuba was one of several rationales that Khrushchev used to justify placing missiles in the island. There was a fear in the Kremlin that if Moscow did not step up its commitment to Cuba that they would fall in line with Beijing as Maoism was more ideologically in sync with Cuban revolutionaries like Che. Khrushchev and other Soviet leaders appreciated that if they came away from the crisis with nothing, the PRC would attack them for abandoning a fellow socialist country. By the same token, they also feared that the Chinese would egg on Castro to a Soviet-US war in which the winner would be an uninvolved China. The US non-invasion pledge was a way to denude the sting of this criticism and served as a means to show the Third World that while Moscow used diplomacy to protect the interests of its allies, Beijing favored reckless adventurism.

The other reason for the pledge emanated from the Cubans themselves. Although there was an assumption in the US that Castro was a Soviet stooge who would toe Moscow's line, he was an exceptionally prickly ally for the Soviets. Castro bristled at the Soviet's unilateral decision to dismantle the missile bases and bend to US pressure. The Soviet agreement with the US called for the dismantling of offensive long-ranged nuclear weapons in Cuba. Both conventional weapons and short-ranged tactical nuclear weapons remained in a grey zone for these agreements. The American position was that jet bombers like the IL-28 as well as cruise missiles with nuclear warheads were part of the deal reached to end the quarantine. The Soviets' initial position was more skeptical and they may have fought the US on this matter, but were forced to the US side by their ally Castro. The Cuban leader was by various reports somewhat apoplectic that he had been locked out of negotiations over the missiles. The Soviets knew that Mao had made overtures to the Cubans during the crisis and that the Cubans were now asserting in the UN and other venues that some of this equipment had been transferred to Cuba.

Khrushchev's personal envoy Anastas Mikoyan went to Cuba in November 1962 to personally smooth over relations with Castro. The Soviet negotiations with the Cubans were not particularly cordial. The US's decision to deal with the USSR and ignore Cuba in this matter rankled with Castro as did Khrushchev's unilateralism during the crisis. Mikoyan, who was one of the leading skeptics in the Kremlin over placing missiles in Cuba in the first place, came away from these meetings convinced that Castro was too much of a wild card to be trusted with even short-ranged nuclear equipment. The Soviet envoy actually invented an unnamed and allegedly secret Soviet law that forbid the transfer of these weapons. Castro eventually buckled under Soviet pressure, but his independent streak meant that the Soviets would be more cautious when it came to their Cuban ally in the future.

The non-invasion pledge in this context suited Soviet diplomatic standing far more than it did for Cuba's. The Soviet backing of the pledge denied Castro an issue which to force the transfer of tactical nuclear equipment to the Cubans and also provided an unspoken warning that Soviet largess was at Moscow's discretion. The Soviets could always threaten to turn the spigot of aid off using the pledge as a pretext if Castro stepped out of line. Globally, the non-invasion pledge also helped ward off charges of being soft on capitalism from Beijing. The pledge was not the Soviets' end game in placing missiles in Cuba. Instead it emerged as a poor consolation prize for a crisis of Khrushchev's own making.

Sources

Domínguez, Jorge I. "The@# $% & Missile Crisis:(Or, What Was “Cuban” about US Decisions during the Cuban Missile Crisis?)." Diplomatic History 24, no. 2 (2000): 305-315.

Fardella, Enrico Maria. "Mao Zedong and the 1962 Cuban Missile Crisis." Cold War History 15, no. 1 (2015): 73-88.

Mikoyan, Sergo Anastasovich, and Svetlana Savranskaya. The Soviet Cuban Missile Crisis: Castro, Mikoyan, Kennedy, Khrushchev, and the Missiles of November. Washington: Woodrow Wilson Center Press, 2012.

Taubman, William. Khrushchev: The Man and His Era. London: Free, 2005.