* To protect its own interests. The US may intervene to protect its own citizens, its economic interests, or its national security. For example, the US intervened in the Dominican Republic in 1965 to prevent the country from falling to communism.
* To promote democracy and human rights. The US may also intervene to promote democracy and human rights in other countries. For example, the US intervened in Haiti in 1994 to restore the democratically elected president.
* To prevent the spread of conflict. The US may intervene to prevent the spread of conflict from one country to another. For example, the US intervened in the Korean War in 1950 to prevent the spread of communism from North Korea to South Korea.
* To fulfill its obligations under international law. The US may intervene to fulfill its obligations under international law, such as the United Nations Charter. For example, the US intervened in the Gulf War in 1991 to enforce UN Security Council resolutions that called for Iraq to withdraw from Kuwait.
In the case of the United States' intervention in the Korean War, the US was motivated by a number of factors, including:
* The threat of communism. The US was concerned about the spread of communism from North Korea to South Korea, and from there to other countries in the region.
* The importance of South Korea to US security. South Korea was considered a strategic ally of the US, and the US was concerned about the potential loss of South Korea to communism.
* The desire to uphold international law. The US believed that North Korea's invasion of South Korea was a violation of international law, and that it was necessary to take action to enforce the law.
Ultimately, the US decided to intervene in the Korean War in order to protect its own interests, to prevent the spread of communism, and to uphold international law.