Final answer:
Robert McNamara developed the 'flexible response' defense strategy to move away from the 'mutually assured destruction' doctrine and to provide the US with a range of military options against various global threats during the Kennedy administration.
Step-by-step explanation:
The part of US defense that Robert McNamara developed was flexible response. This strategy advocated by McNamara during John F. Kennedy's administration was a reaction to the all-or-nothing defensive approach of mutually assured destruction that was favored during Eisenhower's presidency.
The flexible response strategy aimed to enable the United States to respond with a variety of military capabilities to different global situations, from small insurgencies to full-scale war, which included an increase in both conventional and nuclear capabilities. The establishment of special forces like the Green Berets, as well as the increase in nuclear arsenals, were key components of this strategy. McNamara prioritized this approach because it provided multiple defense options and increased the ability of the US to respond more appropriately to the spectrum of threats during the Cold War era.