How did the war end for the US? How did war end for the Vietnamese? How was this possible?

During the 1060?-1975, there was a war called Vietnam War. It was a major war between the US and Vietnam. But as a result, the North Vietnamese defeated the US and united Vietnam. But how was this possible? And how did the war end for both countries?
The Vietnam War was a useless war for the US. The aim of US involvement into the Vietnam War was contains spreading of communism but the consequence of the US involvement was completely fault because the US government couldn't stop the spreading of the communism, rather, they made a origin to Cambodia and Laos turn communism by attacking them…