During the california gold rush of the 1850s some americans in the eastern part of the united states reached california by sailing all the way around south american

The United States feared that if communism was successful on Vietnam, the rest of the countries that border it would soon turn into communism and those other countries that border that new communist country would also be converted to communism and so on. This is called the "domino theory" and made the United States to send troops to prevent the theory from happening.