So the Soviets didn’t actually really do anything to fool him.

The concern that the US was lagging behind the USSR in nuclear missiles mainly came from one report, titled Defence and Survival in the Nuclear Age, also known as the Gaither Report. It was commissioned by Eisenhower in ‘57, and was originally meant to assess simply US defensive systems, mainly fallout and blast shelters, but ended up recommending massive increases in military spending, claiming the USSR had potentially thousands of nuclear missiles.

This report was vastly inflated, using Air Force estimates that were incredibly inaccurate. The reports contradicted directly with CIA reports, which were far more accurate (though still over estimating the USSR capability), and had been using the top secret U2 near-space reconnaissance planes to get a better picture of the USSR capabilities.

Because of this, Eisenhower shelved the report, basically ignoring the recommendations of the Gaither committee. However, the report was leaked to Kennedy in 1958 during his senate run. Kennedy at this time was probably unaware of how inaccurate the report was, and used it in his political campaigns to try and paint the Eisenhower administration as weak on defence. It is during this campaign where the term “missile gap” is first coined.

Eisenhower knew the report was wrong, but refused to publicly refute the claims, as he feared revealing how much they knew about the USSR capabilities might jeopardise the U2’s secrecy. To try and resolve the misunderstanding, Eisenhower arranged for both JFK and Lyndon Johnson to be briefed on the accurate figures.

However, Kennedy continued using the “missile gap” in his political rhetoric into 1960 and his presidential campaign. The historical consensus tends to claim this was because Kennedy himself was often accused of being weak on military issues, and he was finding that his missile gap talking point was benefitting his campaign to the point he felt he couldn’t drop the issue.

A few years later, with the development of satellite imagery, it became even clearer that the USSR had never been close to competing with the US in terms of missile capabilities.

So in short, the soviets never did anything to convince JFK of the missile gap. The original idea came from a US report, which was immediately recognised as hugely overinflated, but was then leaked to Kennedy. Once Kennedy was informed of the truth, his “missile gap” rhetoric was proving so useful to his political campaigning that he continued to claim it was a real issue.