Why did America enter the Korean War in 1950?

Intro - Outline argument and raise questions. 

P1 - Represented a bigger struggle. North Korea's Kim Il Sung told Stalin he believed he could conquer South Korea, and Stalin gave him his backing as a way to indirectly discomfort America. US did not want a full out war with the Soviets, however Korean war was a good way to demonstrate their strength. NSC-68 advocated showing military strength, and a victory in the Korean war would show this.

P2 - Conflicting ideologies. America was involved in an ideological war with Soviet Union, and against the communist government in China. Both of these countries backed North Korea in 1950 as they invaded South Korea. Idea of America standing for the free world and democracy, whilst Truman saw Soviet Union as oppressive. Truman felt duty to stand up to the communist regime. NSC-68 advocated a more aggressive foreign policy to Soviet expansion.

P3 - 'Domino theory'. Truman believed that if one country turned communist, others would follow. Korea was near Japan, which was an important country for US trade, and also a potential strategic location for US forces against China. Churchill 'Iron Curtain' descending over Eastern Europe. US feared this would also happen across Asia if communism was allowed to get a foothold.

Conclusion - They were worried about the spread of communism, and 1950 also represented a change in foreign policy after the NSC-68 - more aggression was shown by the US.