Jinxi I think you have to view this question with the fact in mind that the US has to be concerned about two main areas in their security and they are the seaboards of the Pacific Ocean and the Atlantic.
In WWII most of the US did not wish to be brought into a European war (Atlantic side) and were happy to watch progress or in real terms Allied failure deep into 1941. Churchill and Roosevelt were singing from the same songsheet and I feel there was an inevitability of the US being brought in but it took a direct action of the Japanese (Pearl) to make them respons. This of course was the Pacific side. Germany then declared war on the US because of its agreement with the Japs. So then they were at war on both seaboards. Many historuians feel it was not then if, but when the Allies would succeed.
In Vietnam there was a far east threat which was seen by the US as the spreading of Communist doctrine throughout the world - one aim of that philosophy, and they decided to turn the tide back wards. Unfortunately for them it was a war they did not win. The US will say they didn't lose, maybe with truth, but win it they did not. So I see your question in it's simple form as a battle to stop fascists or communism. Stop action in the greater globe from expansionism by different countries. All of which affected and affects the US economically. And that matters.