See answer below.
To destroy the enemy.
Vietnam was a countryside, with the US military living in it (and the enemy). Civilians had to vacate the area (we RE-LOCATED THEM).
The US. Only the US was bombing North Vietnam. Although American and Australian men 'o war were bombarding North Vietnam along their coastline.
First US war in which an enemy country (nation) won.
Attrition of the enemy, with body counts.
The US military used about 5,000 war dogs (officially designated Military Working Dogs) during the Vietnam War, and approximately 10,000 US servicemen performed the duties of "dog handlers" in the war.
All states were involved. US law does not allow any one state to conduct war; it's all or nothing. If an enemy attacks one US state, that enemy has attacked all US states.
It was the first US war in the enemy nation triumphed. It was a fight against communism.
Underestimating the enemy.
Communists, the NVA, the VC, and protesters in the US (they OFTEN flew the enemy flag in their "peace marches."
The strategy in the Vietnam War was to attrit the enemy. Kill more of them than they could of us. The tactic of "Search and Destroy", which resulted in the process of counting bodies (the body count), became the normal function of the Vietnam War.
When the US withdrew from Vietnam in 1975, it did nothing to repair Vietnam, on the principle that Vietnam was a member of the Soviet bloc, and their communist allies, the USSR and China, were the ones who were responsible for any needed international assistance to Vietnam. It is much easier, from a political viewpoint, for the US to help a defeated enemy, as it did with West Germany and Japan and Italy, following WW II, than it is to help a victorious enemy. After fighting so desperately to prevent communist North Vietnam from taking over South Vietnam, the US was not then going to help communist Vietnam to rebuild after the war. The communists won the war, they therefore could rebuild themselves. Since the end of the Cold War in 1990, relations with Vietnam have improved, but are still not very close. The US does not like to lose.