There could be all sorts of reasons for a country to go to war. The US intervened in the Korean was and the Vietnam War, I think the most important reason they did that is the domino theory.
The domino theory means 'if one falls, all falls'.
Militarily, South-East Asia is a very important place. It is the perfect place to make a navy and air force base to seize the whole pacific. The US already had Pearl Harbor, they couldn't afford to let a communist country, or maybe even their allies have South-East Asia. That would make them lose control of the pacific. …
