On Roman General, Gnaeus Papirius Carbo, Repelling the Advancing Celtic Armies Purely Through a Show of Force (Circa, 112 BC), Only to Be Subsequently Routed After His Treacherous Plan to Ambush the Celts Was Detected

Yeah, this was quite possibly one of the worst military decisions in history in that it led to the deaths of hundreds of thousands of Roman soldiers and damn near crushed its Republic. Fortunately for Rome some superior generals followed this horse's ass but, yeah, this was clearly the greatest threat to the Empire since the second Punic War when Hannibal also attacked from the north.