Why didn't the US just pull out of the Vietnam War?

The Vietnam War was the first US experience of losing a war, that is to say...not gaining a victory; consequently, the US was new at this type of procedure. Therefore, President Nixon tried to withdraw from the war with "honor"; he bombed and invaded Cambodia (1970) and Laos (1971), and negotiated a return of our POW's, and then we departed. Those procedures were necessary, because in part, North Vietnam held probably close to a thousand US Airmen as POW/MIA's.