Since parts of China and North Vietnam were already communist, the United States was afraid that communism would spread throughout Asia. So, they decided to send money, supplies, and military advisers to help the South Vietnamese Government.
Pretty much all abled men of age were drafted into war, especially those of the lower classes.
Of course, wealthier men who had more money would often buy their way out of fighting in the war. Women on the other hand had no such requirements as their male counterparts, and in some cases, they weren't even allowed to join the military.
This wasn't a fair system, because far more men were losing their lives than women, (Not that women are more deserving of dying than men. No one should have to fight in a war against their will.) and only if a man had the money, was he allowed to basically, live.