The domino theory during the post world war 2 era that

The domino theory was the theory that states that all of the countries surrounding a country that fell to communism will do the same

"the theory that a political event in one country will cause similar events in neighboring countries, like a falling domino causing an entire row of upended dominoes to fall." 
If you're referring to the cold war, the reason us entered the war because they wanted to prevent the spread of communism. Korea goes communist, their neighbor goes communist, their neighbor goes communist and so on until it spreads throughout Asia.