The Soviet Union was for communism, and the United States was against it.
In the Korean War, the Soviet Union allied with North Korea, and the U.S. allied with South Korea. They both got involved in the Korean war by helping them fight each other.
people had different views on communism like in Vietnam the Vietcong believed in communism and south Vietnam believed in democracy so the north invade the south and then that started their civil war then the U.S. help the south by using the Truman Doctrine to send troops to Vietnam to help the democratic south
