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. Because of these reasons, in the 40's, the Japanese empire once invaded Vietnam, and that led to the pacific war between Japan and USA. Vietnam alone isn't that a big deal, but the US thought if Vietnam falls into communist hands, then Laos, Cambodia, Thailand, Malaysia, Indonesia etc fill fall like a domino.
The US was determined to prevent this, their position and power over the pacific was too important to be jeopardized in any kind of way. If they lose control over pacific, that could bring wars right into American soil. They had to stop the first 'domino' falling.
In 1954, the US wanted to keep Vietnam under French control; they financed 80%of French war effort in Vietnam. When the French withdrew their forces in Vietnam, US government support the South-Vietnam directly, which was a capitalist country.
Politically, under the Geneva Agreements, 1854, a ceasefire was declared, Laos and Cambodia became independent states, Vietnam was to be temporarily divided at the 17th parallel until national elections could be held within two years and France would withdraw its forces from the region. The North of Vietnam was to be under the...