What's the purpose? Fighting Communism or Killing young men in their prime?

I would like to now how the government of the United States, devoted to democracy and freedom, could allow this. So many men died for what reason, to prevent the spread of communism? I believe that they fought for their own personal individual reasons. Most of their reasons were to defend the land they love; however, not knowing that they would probably die in a war with no real justification.