Why was Vietnam considered a “different kind of war?”

Unlike the second world war, there was moral ambiguity in Vietnam. America faced an ill-defined enemy that was hard to distinguish from civilians. There were American atrocities and massacres of unarmed civilians. It was hard to portray this war as "good" versus "evil", as had been the case during the Second World War.

there was moral ambiguity in Vietnam. America faced an ill-defined enemy that was hard to distinguish from civilians. There were American atrocities and massacres of unarmed civilians. So in this war there was actuality  no real good or bad guy

Explanation:

the congress realized that action had to be.