Can You Explain How The U.S. Became Involved In The Politics Of Southeast Asia?

USA's involvement in Southeast asian politics was mainly because of China's and Russia's involvement in those areas. Both of these super powers were gaining by interfering in those areas and thus USA had to keep check on rivals. Vietnam war was the most popular USA interference in southeast Asia. It was fought mainly for preventing rivals becoming more strong and USA lost much because of that. All the detailed and very to-the-point account of USA's involvement in Southeast Asia is mentioned in detail in link provided below.
Do visit this Detailed but to-the-point article of US involvement in Southeast Asia.