For what it's worth here's an answer I wrote recently on the context and history of US popular support for the Vietnam War.

TL:DR - the American public was supportive of involvement in Vietnam until casualties rose, and this was in part because of perceptions around too many Americans being killed for unclear objectives. The war was sold as limited conflict that could be won decisively and quickly, and both public support and support within the US government fell when this proved to not be the case.

Also the US Department of Veteran's Affairs has a fact sheet for US casualties in major conflicts. 53,000 American battle deaths were suffered in World War I, but another 63,000 died from non-battle-related causes (mostly the flu pandemic).

It's also worth pointing out that the entire US involvement in World War I lasted 19 months, as opposed to the 8 years of official conventional military involvement in Vietnam.