Viet Nam.
The American Armed Forces fought well and defeated their enemy on the battlefield, but was defeated by their own people in the homeland - the politicians, the protestors, and the timid American people - and it exhibited a kind of weakness that still haunts this nation today.
Our enemies of today believe that America is a "paper tiger", and with good reason; as long as they continue to bleed us little by little, waging a war of atrition, a death by a thousand cuts, we will eventually cut and run like we did in Viet Nam, Beirut, and Somalia. Showing weakness only emboldens our current enemies, the Islamofascists only understand power, we must not back down!