In all of these wars, the soldiers believed they were fighting for their own personal freedom, or the freedom of some other group of individuals. Had they not believed this, they would not have risked their lives. War it a last resort. Generally speaking, people do not like war, and want to avoid it at all costs. It is believed that a large part of why America went to Vietnam was for monetary reasons.
Companies have to make all of the ships, helicopters, airplanes, and supplies.
As a result, jobs are created and the economy in stimulated, but the U.S. government could never tell that to the public. It is very possible that these were ulterior reasons for the Vietnam war, but the government simply turned it into an issue of freedom. As a result the public was told that after trying to make peace with North Vietnam, the U.S. Navy was continually being assaulted and that there was no option but to go to war. At that point Americans had the option to believe that they were involved for noble and acceptable reasons. Even though freedom of some type is directly related to all major conflicts, it is not necessarily the freedom of all those involved.