Briefly describe Domino theory and how it affected US foreign policy.

Domino theory was an idea in the US, popularised during Eisenhower's time in office, that stated that if a country fell to communist forces, then other countries around it would also fall in succession, as a result of a sort of domino effect. Therefore this led to the idea that the US should intervene early against communist movements, even in nations where they were popular, in order to stop the spread of the ideology.
This shaped US foreign policy as it contributed to the decision to intervene in various ways, most notably in Vietnam and Cuba, where there was direct aid and military assistance given, but also in other places like South America and Indonesia where more covert methods were used.