Answer:
The 25th amendment was enacted twice during the Nixon administration because, even though the letter wasn't legal in 1955 Eisenhower had a heart attack and Nixon became acting president.
Explanation:
Because a while after that Eisenhower had to have surgery in 1965, and Nixon became president for a period of time.
The flexible response was created and implemented during former President John F. Kennedy's administration at the year 1961 as a measure to counteract the New Look policy of former President Dwight Eisenhower in his policy of conducting a massive military action in case of war.
