with negotiations that removed Soviet missiles from Cuba and U.S. missiles from Turkey
It is said that the Cuban Missile Crisis of 1962 was the closest the Cold War ever came to erupting into an actual nuclear war between the superpowers.
What was the main reason for U.S. involvement in Vietnam?
the belief that communism would spread in Southeast Asia
This was known as "domino theory" -- that the spread of communism in Vietnam would result in other countries in the region falling like dominoes to the influence of communism.