Where can I find reliable sources for why the U.S lost the Vietnam War?

a href="http://www.learnhistory.org.uk/vietnam/usdefeat.htm" rel="nofollow"http://www.learnhistory.org.uk/vietnam/u…/a a href="http://www.vietquoc.com/whylost.htm" rel="nofollow"http://www.vietquoc.com/whylost.htm/a a href="http://www.thirdworldtraveler.com/Asia/Vietnam_War_US_Lost.html" rel="nofollow"http://www.thirdworldtraveler.com/Asia/V…/a a href="http://www.socialistworker.co.uk/art.php?id=459" rel="nofollow"http://www.socialistworker.co.uk/art.php…/a a href="http://urbanlegends.about.com/library/bl_general_giap.htm" rel="nofollow"http://urbanlegends.about.com/library/bl…/a a href="http://wiki.answers.com/Q/Why_did_the_US_lose_the_Vietnam_War" rel="nofollow"http://wiki.answers.com/Q/Why_did_the_US…/a a href="http://www.allfreeessays.com/essays/Lost-War-Vietnam-War/1989.html" rel="nofollow"http://www.allfreeessays.com/essays/Lost…/a