The tale of the Persian nobleman Zopyrus appears in the third book of Herodotus, from chapter 150 to 160.  We will relate it here, with no justification beyond the insight it may provide on human nature. 

During the reign of the Persian king Darius I, the Babylonians staged a revolt against his rule.  Darius besieged Babylon unsuccessfully for many months.  Darius’s prospects did not seem to be improving with time, and it was apparent that the venture would fail unless something changed significantly.  From the elevated walls, one defender even taunted the Persians, saying that the city would only fall “when mules bear young.”  Since mules are sterile animals, this was a prosaic way of saying that the city would never be captured.     

But twenty months after the siege began, a patriotic Persian nobleman named Zopyrus noticed that one of his mules had miraculously produced a foal.  This for him was an omen predicting the imminent fall of Babylon, and he resolved to craft a scheme to expedite this very event.  Eventually he settled on a ruse that first called for him to mutilate himself grievously.  So Zopyrus cut off his nose and ears, flagellated his back without mercy, and shaved his head.  In this condition he requested an audience with King Darius.  The king, of course, was horrified by the wretched spectacle standing before him. 

But Zopyrus calmed the regal nerves, explaining to the king that he had harmed himself for a reason.  He then related to the shocked Darius exactly what his plan was.  Somewhat reluctantly, the king gave his assent to the scheme.  Zopyrus would present himself to the enemy as a deserter, and then slowly seek to take command of Babylonian defenses.  He told Darius to wait until ten days passed after he arrived at Babylon, and then station by the gates of Semiramis a force of a thousand men he could afford to lose.  Seven days after this, he told Darius, the king should send two thousand more men to the gates of Nineveh; then, twenty days later, he should send four thousand men outside the Chaldean gates.  These soldiers should be only lightly armed.  Zopyrus finally instructed that a general assault on Babylon from all directions should be ordered twenty days after the four thousand men were sent to the Chaldean gates.   

Zopyrus was counting on the fact that the Babylonians would accept him as a deserter, and entrust him with a role in defending the city.  He then appeared before the walls of Babylon and deserted to the Babylonians, claiming that the Persians had viciously tortured him; as proof he offered the obvious condition of his own body.  Zopyrus launched into a tirade against Darius, and expiated at length on how cruel he was; he promised his hosts that he wanted to help them defeat the king.  The Babylonians believed his story, or at least accepted it, since their hatred of the Persians had made them blind to reason and good judgment.  It never occurred to them that a man could be so devoted to his cause, and to the cause of his country, that he would be willing to mutilate himself to accomplish a larger strategic objective.

The Babylonians agreed to allow Zopyrus to participate in the city’s defense.  In accordance with his prearranged plan with Darius, Zopyrus ten days later led a detachment of soldiers to attack the Persians stationed outside the gates of Semiramis, and easily crushed them.  Seven days after this he led a Babylonian force to easy victory against the two thousand men camped outside the gates of Nineveh; and twenty days later he did the same thing against the four thousand men sent outside the Chaldean gates.  All of these “victories” were, of course, foregone conclusions.  But the Babylonians believed they were legitimate demonstrations of Zopyrus’s military skill and credibility.  So confident were they in Zopyrus’s abilities, that they were prepared to entrust him with the defense of their entire city.  In fact, they appointed him to the positon of “Guardian of the Wall.” 

Yet everything was soon to change.  On the agreed day, Darius ordered an assault on Babylon from all directions, as Zopyrus had instructed long before.  When this attack began, Zopyrus opened two gates, the Cissian and the Belian gates, and allowed the attackers to swarm into the shocked city.  The inhabitants realized, too late, that they had been betrayed, and finally grasped the extent of their folly.  Babylon was captured, and its walled defenses were eventually pulled down.     

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Read more about stratagems of war in the new, annotated translation of Frontinus’s Stratagems.