I assume that you are asking why some Americans opposed the war. I have changed your question to reflect this.
Many Americans who opposed the war did so because they felt that it was not a war that was necessary for the security of the US. They felt that what happened in Vietnam could not truly impact the US. Other opponents of the war opposed it because they believed that the US was trying to oppress the Vietnamese. These leftists believed that communism was a better way than capitalism. They felt that the US was a country that was acting aggressively in order to try to promote its own ideas even if those ideas were bad for Vietnam.