Please be aware of the Forum Rules of Conduct.
Technically America didn't "lose" anything, military-wise. The Tet Offensive wiped out the vietcong, and the NVA was in tatters as well. Unfortunately, this is where civilians began protesting the war, leading to a situation where a military victory strategically was turned into a defeat morale-wise by the sensationalist media and the counter-culture movement. This is one of the reasons that Sun-Tzu said that war fighting should be left in the hands of the generals once it is declared. Nothing stifles a military campaign more than outside intervention by civilians who most of the time have no clue as to what they are doing.