The default position for those who are opposed to America projecting our power is Vietnam. But the real lesson that should be taken from Vietnam, and Korea, is that if there is no stated goal of what would be a victory then military intervention is fruitless. In Vietnam the goal was ????????, so defeat was the abject withdrawal of our troops and abdication to the North Vietnamese communists. In Korea the goal was containing the North Koreans to the 58th parallel; we're still doing that 50+ years later.
In Afghanistan the goal must be the destruction of the Taliban as a political force. If we can't do that then all that has been ventured is a waste. Afghanistan is difficult both culturally and geographically, but with political commitment to the military, the goal can be obtained.