I would have to say the failure to acheive victory in Vietnam.

Not only did it screw the people of Veitnam, but also the Cambodians. This tragety aslo lead to many state dept. failures and really hurt the American pysce for years to come. But most importantly the effect it had on our Armed Forces and those that serve. It is sad IMHO what we did to them in country and then when they returned.

And for what? This conflict was fought based on politics not on the urgent need to win and come home.