A "just" war is often defined in terms of the following criteria: first, it must have a just cause, such as self defense against an aggressor, and not merely be a way to grab another country's goods or territory. Second, diplomatic avenues must first be exhausted before a country turns to war. Third, it needs to be declared and waged by a legitimate group (a government rather than terrorists), it needs to be winnable, so as not to simply bring suffering and disaster on a group of people, and it needs to be proportional, meaning, for example, a country should not use nuclear weapons when its purpose can be achieved with tanks and machine guns.

Under these criteria, it is difficult to justify the Vietnam War as a just war. The US government got involved in a war with a small, weak country halfway around the world not because it posed a direct threat, but in the interests of stopping the spread of communism. The US escalated the war because it hoped for a quick and decisive win due to superior military capability. Public opinion turned against the war because it became increasingly obvious that it was being mismanaged, that a quick, decisive victory was not going to occur, and that young American men were being sacrificed for reasons that seemed dubious and unclear to many Americans. There seemed to be no urgent reason for the US to be fighting in Vietnam.

The government responded to criticism by first doubling down on the war effort. As this did not work, President Johnson's response was to decide not to run for reelection, while Nixon campaigned on the promise of negotiating a quick peace. This proved to be far more difficult than anticipated, leading to the war dragging on and the Nixon administration engaging in secret campaigns in Cambodia and Laos.