The correct answer is to stop the spread of communism.
The North Atlantic Treaty Organization (NATO) was formed shortly after World War II. This organization that consisted of the US and their allies was formed as a group of countries dedicated to fighting the spread of communism. Countries within this organization agreed that if one country was attacked by the communist Soviet Union, all of the other nations would send support.
The Southeast Asia Treaty Organization (SEATO) was another agency with the same idea. The US and several southeastern nations (like the Philippines and Thailand) worked together to stop the spread of communism in that region.