The Vietnam War was not considered a just war for several reasons:
1) President Lyndon Johnson launched an air attack on North Vietnam without a formal declaration of war or permission from Congress, violating the Constitution.
2) The widespread bombing campaigns and use of defoliants like Agent Orange caused massive civilian casualties and long-term health and environmental issues for Vietnamese people.
3) There was no sufficient or proportional cause for the United States to intervene militarily in Vietnam's civil war between North and South.