The USA is a large country with such a diverse population that when it went to war it was inevitable that there would be such mixed reactions from the American public. Although, to begin with, a lot of the Americans agreed with the war because it stood for a common belief in America at the time - that communism was evil and that the country should do whatever it could to protect itself and other countries from the domino effect. They believed it was their patriotic duty to go to war. Not only did they think the country should go to war they thought that it would be over in a short time and with the country never having lost a war the American public had every right to feel confident. In the 1950's and 1960's America was very confident and were convinced that its economic and military power would be enough to sway other countries away from communism, just like the Marshal Plan had done in western Europe.
President Johnson told people that to prevent a bigger issue and a bigger war the country would need to fight a small war. Therefore the American people believed that the Vietnam War would be a small war and the country needed to fight a small war in order to prevent a larger war. Therefore the Americans entered war with very high morale, however this soon changed due to a number of different reasons.
One of the reasons why some American people didn't agree with the war was because they were afraid of more young men being killed. The US army had already suffered quite a severe loss a few years before in Korea where 30,000 men were killed and the Americans didn't want to see any more of their men killed.