Although these are synonymous and can be interchanged, it would seem that debacle has more uses. As I found in the Oxford Dictionary; Fiasco - 1. A sudden and violent collapse. Debacle - 1. A sudden and violent collapse. 2. A sound defeat. 3. Flooding caused by a tumultuous breakup of ice in a river during the spring or summer. Further reading; http://sentence.yourdictionary.com/fiasco http://www.vocabulary.com/dictionary/debacle