This wasn’t so much an agreement as it was already part of our scheduled deployment plan. The PGM-19 ‘Jupiter’ IRBMs deployed in Turkey and Italy in 1961 strictly as a stopgap measure until the SM-65 ‘Atlas’ and SM-68 ‘Titan I’ could be fully deployed. By mid-1963 all planned deployment of these systems was complete and the Jupiter and PGM-17 ‘Thor’ missile deployed in the UK were withdrawn from strategic service, although they were pressed into service or salvaged into families of space launch vehicles (SLV)[sup]*[/sup] The cryogenic fueled Atlas and Titan I were themselves rapidly replaced at that point by the more reliable and capable storable liquid LGM-25C ‘Titan II’ and solid motor LGM-30A/B ‘Minuteman I’, and the modified Atlas went on to become a significant heavy SLV including the manned Project Mercury orbital flights.
So we actually gave up nothing by this ‘concession’ and had we actually understood what was driving Khrushchev to deploy missiles in Cuba (i.e. saving face to hardliners in the Politburo and the military establishment who were perhaps legitimately concerned about the strategic threat the IRBMs represented) we possibly could have resolved the conflict without very nearly coming to blows. As it was, the information we had–that the SS-3 and SS-4 systems being deployed in Cuba were not ready for prime time (true), and that nuclear weapons were not already delivered onto Cuban soil (false)–convinced Kennedy to bluff his way into a blockade and confrontation, which pushed Khrushchev into a corner. Only the advice of Llewellyn “Tommy” Thompson, who had been Ambassador to the Soviet Union and knew Khrushchev personally, persuaded Kennedy to respond to a conciliatory message from Khrushchev and not to a later bombastic one (probably drafted under the eyes of military advisers), agreeing to a back-down from positions and the tacit agreements of bilateral removal of missiles. This was probably one if not the key factor in Khrushchev’s later being removed and replaced by Leonid Brezhnev who recanted the economic reforms and political liberalization that occurred under Khrushchev, replacing the former’s progressive openness with the West with passive détente and stagnation.
As for going to Defcon 2 (which we never fully did, owing to C[sup]3[/sup] limitations of the era), it didn’t make a significant difference. The Soviets knew that we’d respond to an attack, and had planned accordingly. Castro (at least after the fact) claims that he advised the Soviet Union to attack with nuclear weapons should an invasion of Cuba by US troops occur, knowing full well that Cuba would have been utterly annihilated. There was no question of willingness to go to war; the only question was who blinked first. You could argue that the US ‘won’ (i.e. we didn’t give up anything material), but we lost the opportunity to defuse Cold War tensions and encourage liberalization in the Soviet Union and Warsaw Pact, so it was a bit of a Pyrrhic victory in that sense. Honestly, everybody walked away from the Cuban Missile Crisis alive which was better than any other alternative. The only party that could truly be said to have gained anything was Cuba, who became the avatar of permanent Soviet presence in the Western Hemisphere and received a lion’s share of foreign aid because of this status.
[sup]*[/sup]The Jupiter became the basis for the Juno II liquid SLV (not to be confused with the Juno solid propellant target rocket) and the core of the Saturn I and Saturn IB S-1 first stage, and the Thor mutated into the Thor-Able and Thor-Delta SLV with the addition of the Able (sometimes called Ablestar, later Delta) second stage derived from the failed but innovative Vanguard program, and a variety of solid third stage/kick motors, plus the Thor-Agena and Thrust Augmented Thor. Thor-Delta later became known as just Delta, and was the basis for the Douglas (later McDonnell Douglas, later Boeing) Delta I, II, and III families of vehicles. (The Delta IV uses a completely new Common Booster Core amd RS-68 motor that essentially shares no history with Thor and previous Delta systems.)