Explain the key difference between the Truman Doctrine and the Eisenhower Doctrine?

The Truman Doctrine was an American foreign policy during the presidency of Harry Truman that seek to support the free peoples that were struggling and resisting the attempts of subjugation by armed minorities or foreign pressures. This policy was launched in 1947, at the beginning of the Cold War, as a way to stop the dissemination of Communist regimes supported by the Soviet Union that threatened American hegemony and capitalism.

The Eisenhower Doctrine or Massive retaliation, on the other side, consisted of a nuclear and military strategy during the Cold War, that stressed the use of nuclear weapons in order to deter the opponent to commit certain actions or attacks. It emphasized a disproportionate retaliation in case of an attack in order to make an effective deterrence.

So, the basic difference was that the Truman Doctrine put the key factor on supporting allies and resisting groups against the Soviet Union, while the Eisenhower Doctrine put the key on nuclear weapons as a way of deterrence.