massive arms buildup
His response was an embargo.
first strike capiblity
He ordered a naval blockade of Cuba.
either a build up of nuclear weapons, or a buildup in conventional weapons
Buildup of the Military-Industrial Complex
The USSR responded to JFK's request for a military buildup with a show of strength, particularly through the deployment of nuclear missiles in Cuba, which heightened tensions during the Cold War. Instead of directly confronting the U.S. military buildup, the Soviet leadership, under Nikita Khrushchev, sought to establish a strategic balance and deter U.S. actions by positioning missiles closer to American territory. This ultimately culminated in the Cuban Missile Crisis of 1962, where both superpowers faced the threat of nuclear confrontation.
began construction of the Berlin Wall
The Reagan administration committed the US to protect, serve, and benefit anyone of means. Any citizen with wealth was given rights and opportunities not afforded to anyone else.
Kennedy maintains that the purpose of the Soviet military buildup in Cuba was to enhance Soviet strategic capabilities and project power in the Western Hemisphere. He viewed it as a direct threat to U.S. national security and a challenge to American influence in the region. The buildup was also seen as a means for the Soviet Union to support its ally, Cuba, and counter U.S. actions in Europe and elsewhere. Ultimately, Kennedy believed that this military presence aimed to intimidate the U.S. and alter the balance of power in favor of the Soviets.
The flexible response was for responding to war with flexible whatevers
Two were established, an economic boycott in February of 1962 and the Missile Crisis to prevent the introduction of Soviet weapons in October of the same year. While it is not completely perfect, I suggest seeing the Costner film (based on books about and tapes of the administration) titled Thirteen Days. It is the quickest way to get a good idea of the tension and the situation. The blockade (quarantine) during the missile crisis was because Cuba was (viewed as) a pawn of the USSR and they placed rocket-powered nuclear warheads on Cuban soil aimed at the USA. The blockade was a compromise measure by the Kennedy administration between invading Cuba and thus setting off a likely nuclear exchange with the USSR due to treaties and (being seen to be) doing nothing and being an appeaser to the USSR. Bobby Kennedy worked very hard to handle the politics in the USA and to get buy-in from the cabinet and other stakeholders in the government to avoid military action. The Blockade prevented further buildup of offensive arms aimed at the USA in the western hemisphere and gave the USA and USSR time to negotiate and it gave the USSR time to dismantle the in-place missiles and to save face. Essentially it was a strategic response to an aggressive move by the USSR in the cold war.