The outcome of the Vietnam War was humiliation for the United States. America had originally entered the war to prevent the Communist North Vietnamese and their guerrilla allies in the South from taking over the whole country. Despite the vast expenditure of blood and treasure, that aim was never realized. And so a war that had cost the lives of over 58,000 American servicemen and around 600,000 Vietnamese, Cambodian, and Laotian civilians had ultimately been for nothing. The Communists had won, and proceeded to establish a one-party dictatorship, which still persists to this day.
The significance of the Vietnam debacle was that it showed the limits of military power as a means of halting the spread of Communism. Then as now, the United States had the largest and most technologically-advanced armed forces in the world and yet was still unable to defeat a leftist insurgency highly trained in the art of guerrilla warfare. From now on, the United States would have to be more flexible in its methods of conducting the Cold War, avoiding direct armed intervention where possible and relying on proxies to do its bidding.
On the domestic front, the Vietnam War and its aftermath had a damaging effect on the nation's self-confidence, prompting an unprecedented bout of national soul-searching. To many, it seemed that an era of American greatness was at an end, and that the United States, despite its enormous wealth and military power, was no longer able to impose itself upon the world as it once had.