As a first cut and being very telegraphic:
1. Vietnam 1965
2. Cambodia 1969-70
3. Chile 1973
4. Iraq 2003
5. Nicaragua and El Salvador 1980s
Number 5 is a series of decisions (or course of policy) rather than a discrete decision. Same for what might be my number 6, the backing of the resistance to the Soviets in Afghanistan and then forgetting about the country after that (until 9/11). Number 7 might be the failure to stop the Rwandan genocide.
Update to the list: The Bay of Pigs (as Hank mentions) and subsequent Cuba policy. Some might want to throw in the failed Iranian hostage rescue mission, but that was a question more of implementation/execution or just bad luck, I think. Open to correction though. Then there are the omissions rather than the acts, e.g., failure to do anything very effective about al Qaeda until after 9/11.