In the late 19th century, philosopher Karl Marx articulated a political system called "Communism" which hinges on the pillars of no private property and no religion. It is a step-child of the French Revolution.
Marx predicted that Communism would eventually engulf the entire earth one nation at a time. Eisenhower later dubbed this phenomenon the "falling domino principle." Americans regarded it a "cancer" on the face of the earth.
As a result of the "Bloody Revolution" in 1917, Russia became the first Marxist/Communist nation, led by Vladimir Lenin.
In the upper right corner of the slides below, click to fullscreen. Then click through the following slides one at a time to see the spread of Communism after 1917 marked by the change to Red; Mongolia in 1921; the Soviet Union, Eastern Europe, North Korea (1948), China (1949), North Vietnam (1954), and Cuba (1959).
It is 1948; it sure does seem that Marx's prophecy of worldwide spread is coming about. Note that the Korean peninsula just north of Japan has turned Red by 1948. But the southern portion of that peninsula is NOT Red (yet). China, the most populated nation on earth, would fall to the Communists in 1949.
You are concerned about the falling dominoes, but you don't believe that you can exterminate all communist nations. You have stated that you think the best approach is to "contain" Communism (stop it's spread), rather than try to eliminate it. This policy of containment is called "the Truman Doctrine."
It is 1950. North Korea has invaded South Korea. It looks like South Korea (aka the Republic of Korea) is the next likely place for Communism to spread.
DECISION: You are President Harry Truman. What will you do about the invasion of South Korea?
Click here to find out what Truman did