The idea of "flexible response," initiated by the Kennedy administration in 1961, was to be able to respond to to military aggression in ways appropriate to each situation. Flexible response meant using conventional weapons and methods as appropriate rather than relying on a nuclear arsenal as the USA's primary deterrent in dealing with adversaries.
This was a reaction over against the brinkmanship that had developed under the Eisenhower administration, with the threat of massive retaliation against anyone who threatened the US. John Foster Dulles, Secretary of State under President Eisenhower, had developed the strong approach, going further than the containment policy of the previous administration (of Pres. Truman). In 1956, Dulles described brinkmanship as "the ability to get to the verge without getting into the war is the necessary art." He wasn't afraid to threaten massive retaliation against communist enemy countries as a way of intimidating them.
