What was the biggest threat to america's national security, according to foreign policy experts in both the truman and eisenhower administrations?

President Harry S. Truman in 1947 presented the Truman doctrine, It provided economic and military aid to free nations which were threatened by external or internal opponents i.e.Greece and Turkey. While President Dwight. D. Eisenhower supported Britikmanship which was a practice of using massive military retaliation to threaten an enemy or any aggression.
The U.S reduced its navy and army and expanded its airforce that delivered bombs and built nuclear weapons.