From the same 1961 inaugural as the more famous ask-not line, this sentence laid out Kennedy's approach to the Soviet Union at the height of Cold War tension. It was written during a period when many in Washington viewed any talk with Moscow as appeasement.
Kennedy and Sorensen used chiasmus to hold two ideas in balance without ranking them: do not bargain from weakness, but do not refuse to bargain at all. The structure itself models the diplomatic balance the sentence describes.
Speechwriters often consider this the more sophisticated of the two inaugural reversals. It uses two repeated word pairs instead of one, weaving never, fear, and negotiate into a tighter knot.
Source
jfklibrary.org/learn/about-jfk/historic-speeches/inaugural-address