Throughout the presidency of Lyndon B. Johnson, the Cold War derived from differences of ideologies between countries. The USSR was under communist system, which opposed Capitalist US and most of the Western Europe. Communism had continually spread through Eastern Europe, even threatening Greece, Italy and France, America (Cuba and Mexico since 1960s'), North Africa (Libya in 1970's) and even to the Far East. It began in 1945 when the USSR occupied the Buffer States after Yalta Conference. The US started worrying about countries falling to communism. People including Churchill from his Iron Curtain Speech in 1946 accentuated this. The US took strong actions in the Berlin crisis from 48~49, set up NATO for defense. However, in October 1949, China surprisingly became a communist state, and influenced Korea at the same time. This ended up with a war, Korean War, from 1950 to 1953.
The US couldn't successfully stop it spreading; only South Korea under 38th parallel was saved.
In 1954 the US President Eisenhower talked of countries falling to communism one by one like dominoes. This view that, if one country fell to communism, neighboring states would follow became known as the domino effect or theory.
In 1954 when French withdrew its soldiers from Vietnam after been attacked by Vietminh, especially Dien Bien Phu in 1954, Vietnam was divided into two between 16th parallel. North was under Ho Chi Minh's communist system and South under Diem's government. North could attack South at anytime which meant to the US another domino was falling in Asia after China and North Korea. As mentioned in Eisenhower's theory, this will influence neighboring countries including Cambodia, Laos, Thailand, and even strategic Malacca Cane, which consist of Singapore, Malaysia and Indonesia. During Eisenhower's presidency from 1953 to 1961 massive amounts of economic aid were supplied...