Question

What was the most fundamental difference between the Truman Doctrine and the Eisenhower Doctrine?

Answer 1

Answer: The Eisenhower Doctrine promised military support as well as economic aid to countries fighting communism.  The Truman Doctrine had focused on economic aid.

Detail:

The Truman Doctrine was first stated by President Truman to Congress in 1947, and was mostly about providing economic aid.  Truman stated:  "It must be the policy of the United States to support free people who are resisting attempted subjugation by armed minorities or by outside pressures."  Essentially, the Truman Doctrine pledged American aid elsewhere in the world to check the spread of communist and Soviet influence.  The policy was first put into action in 1948 by providing economic support to Greece and Turkey to stave off communist movement in those countries.

The Eisenhower Doctrine was proclaimed by President Eisenhower in 1957.  Eisenhower pledged to provide economic or also military aid, as needed, to any Middle Eastern country needing help in resisting communist aggression.  This was in response to Soviet and other communist support to Egypt, and as a warning to the USSR in regard to the Suez Crisis of 1956.