In the Year of our Lord 390, the Roman Emperor Theodosius I exacted retribution on the citizens of Thessalonica for an uprising. The Church Father Theodoret recounts what happened:
The emperor was fired with anger when he heard the news, and unable to endure the rush of his passion, did not even check its onset by the curb of reason, but allowed his rage to be the minister of his vengeance. When the imperial passion had received its authority, as though itself an independent prince, it broke the bonds and yoke of reason, unsheathed swords of injustice right and left without distinction, and slew innocent and guilty together. No trial preceded the sentence. No condemnation was passed on the perpetrators of the crimes. Multitudes were mowed down like ears of grain in harvest-tide. It is said that seven thousand perished.
St. Ambrose of Milan heard of the massacre and forbid the Emperor from entering the Church. The entire account can be found in Theodoret, but in part he says:
Fired with divine zeal the holy Ambrosius exclaimed “Rufinus, I tell you beforehand; I shall prevent him from crossing the sacred threshold. If he is for changing his sovereign power into that of a tyrant I too will gladly submit to a violent death.” On this Rufinus sent a messenger to inform the emperor in what mind the archbishop was, and exhorted him to remain within the palace. Theodosius had already reached the middle of the forum when he received the message. “I will go,” said he, “and accept the disgrace I deserve.” He advanced to the sacred precincts but did not enter the holy building. The archbishop was seated in the house of salutation and there the emperor approached him and besought that his bonds might be loosed.
“Your coming” said Ambrose “is the coming of a tyrant. You are raging against God; you are trampling on his laws.” “No,” said Theodosius, “I do not attack laws laid down, I do not seek wrongfully to cross the sacred threshold; but I ask you to loose my bond, to take into account the mercy of our common Lord, and not to shut against me a door which our master has opened for all them that repent.” The archbishop replied “What repentance have you shown since your tremendous crime? You have inflicted wounds right hard to heal; what salve have you applied?”“Yours” said the emperor “is the duty alike of pointing out and of mixing the salve. It is for me to receive what is given me.” Then said the divine Ambrosius “You let your passion minister justice, your passion not your reason gives judgment. Put forth therefore an edict which shall make the sentence of your passion null and void; let the sentences which have been published inflicting death or confiscation be suspended for thirty days awaiting the judgment of reason. When the days shall have elapsed let them that wrote the sentences exhibit their orders, and then, and not till then, when passion has calmed down, reason acting as sole judge shall examine the sentences and will see whether they be right or wrong. If it find them wrong it will cancel the deeds; if they be righteous it will confirm them, and the interval of time will inflict no wrong on them that have been rightly condemned.”
In a letter to the Emperor, St. Ambrose says:
Should I keep silence? But then my conscience would be bound, my utterance taken away, which would be the most wretched condition of all. And where would be that text? If the priest speak not to him that erreth, he who errs shall die in his sin, and the priest shall be liable to the penalty because he warned not the erring.
Ambrose believed that it is the duty of a priest to correct all those in error, up to and including the Emperor of Rome, lest the priest become responsible for not speaking the truth to him. He is referring to Ezekiel 3.18:
If I say to the wicked, ‘You shall surely die,’ and you give him no warning, nor speak to warn the wicked from his wicked way, in order to save his life, that wicked person shall die for his iniquity, but his blood I will require at your hand.
This is the testimony and example of the ancient Church handed down to us as a pattern of how to deal with tyrants who massacre their own people.