Marcus Licinius Crassus

Portrayed as a lean mean fighting machine in Spartacus season 3, which is currently running, Crassus may have been much less an athletic figure.  Known as the richest man in the Roman world, he financed Julius Caesar and became co-ruler of the Republic with Caesar and Pompey later.  His victory over Spartacus' undisciplined slave army may have helped his career, but it gave him an unrealistic view of his military prowess. This brought him to grief later in life, at the age of 60 he set forth from Rome with plans to conquer Parthia in the Middle East, ignoring an extraordinary series of what an objective observer would recognize as really bad omens.

Two years later, ignoring the offered assistance of the allied Armenian army, which had experience dealing with the horse-riding Parthians armed with compound bows, Crassus made wrong decisions every step of the campaign, leading to his death and those of 20,000 Roman soldiers with an additional 10,000 captured.  This event and the loss of Legionary Eagle standards humiliated the Romans and ensured they would be back for vengence.  It also broke apart the delicate political alliance that held together the Republic, after this the antagonism between Caesar and Pompey could not be glossed over, and civil war and the end of the Republic was the result.  

This incredible loss against an inferior force showed the limitations of the Roman military technology and strategy in dealing with enemies on the eastern frontier.  It also showed the bad side of the Roman tendency to give military commands to influential politicians who had miminal or no army experience.  They continued to have problems with the Parthians for centuries and never fully conquered them or succeeding dynasties, thus creating the first solid boundary stopping their previous expansion.