Use debacle to refer to a fiasco, disaster, or great failure. If several dogs run onto the field during the big baseball game, tripping players and chewing up the bases, you can call the whole event a debacle.
Debacle is sometimes used to describe a military defeat. If your army retreats, that's one thing. If your army is outmaneuvered and ends up huddled in a valley, surrounded on all sides by the enemy, forced to sing 90s sitcom theme songs by their savage captors — that's a debacle. Debacle comes from the French débâcler, "to clear," — the prefix des- means "completely, utterly" and the verb bacler means "to block."