What is the "domino theory?" How did this theory influence American foreign
policy in the 1960s?

The domino theory also called domino effect is a geopolitical theory that was prominent in the United States from the 1950s to the 1980s which posited that if one country in a region came under the influence of communism, then the surrounding countries would follow in a domino effect.

This theory influenced american foreign policy in the 1960s after World War II according to which the “fall” of a noncommunist state to communism would precipitate the fall of noncommunist governments in neighbouring states.

example.) the domino theory was used to justify American involvement in both the Korean War and Vietnam War from the 1950's to the 1970's. Both countries are in the South Pacific and both saw Soviet-style communism spread into their political systems.