The Americans kicked the Viet Cong's butts in the Tet Offensive. The Americans did not win the Vietnam War, and the British did not conquer Afghanistan.
The British most definitely DID conquer Afghanistan.
Then they decided it wasn't worth having.
"In 1841 the Afghans rose against the British in Kabul, killing both British agents and surrounding the British garrison. In early 1842 the garrison surrendered, and was offered safe conduct to return to India. However, the British force (made up mostly of the 44th East Essex regiment) was harassed down the Kabul River gorge and massacred at the Gandamak pass before reaching the besieged garrison at Jalalabad.
In retaliation, the British reinvaded with their large Indian army, relieving the British Jalalabad garrison, and then pushed on to Kabul. Ninety-five prisoners from the earlier massacre were rescued, and the British destroyed the citadel and central bazaar of Kabul. However, as Shah Shuja had been assassinated by this point, the British East India Company decided it was unprofitable to occupy the country and withdrew, only after wiping out virtually all Afghan resistance in Kabul."