The Gemara states that a decree of the gentiles is temporary and we do not nullify a Rabbinic decree because of a temporary decree.
In truth, the gentiles are always issuing decrees against the Jews, but HaShem, through His Holy Torah, empowered the Rabbis to institute eternal decrees. This demonstrates the power of the Rabbis throughout the generations. This ideal is evidenced in the following incident quoted in the Medrash (Koheles Rabbah 9:3):
In his continuing dialogue with the great Rabbi Yehoshua, the Roman emperor enjoyed challenging the scholar with questions or other puzzling things. Rabbi Yehoshua would invariably answer him and turn back his challenges, much to the admiration of the intellectual Roman.
One day, as the two were sitting together in the garden, the Caesar turned to Rabbi Yehoshua and said, “You know, when you consider it carefully, I am greater than your greatest prophet, Moshe, the son of Amram.
Rabbi Yehoshua looked at him in surprise. “Why do you say such a thing, Caesar?”
“Very simple. After all, Moshe is dead all these years and I am alive. And did not your Shlomo HaMelech, the wisest of men, say, A live dog is better than a dead lion’?”
Rabbi Yehoshua heard Caesar’s words and said, “I understand what you are saying. Nevertheless, I will show you how wrong you are when you think that you – merely because you are alive – are greater than the great Moshe, our teacher, merely because he is dead.”
“Very well,” exclaimed the Roman. “I am willing to be shown that I am wrong.”
“If this is so,” said Rabbi Yehoshua, “and you are really so great, then surely your servants will listen to you.”
“Naturally,” laughed the emperor. “I am the Roman Caesar and my word is law. Whatever I command will be done without the slightest question.”
“That is good,” responded Rabbi Yehoshua, “because I desire you to order them to do something and see if they will obey your commandment to the letter.”
The Roman smiled tolerantly. “I just told you that they will do anything that I ask of them.What shall I command them to do?”
“Command them that for three days they shall not light fires in all their dwelling places. If you have this order sent to them and, if they indeed listen to you, then I will admit, also, that you are greater than Moshe, the son of Amram, who was our greatest prophet.”
The Roman emperor laughed aloud when he heard Rabi Yehoshua’s words.
“Surely, you are joking. Is this a difficult thing you are asking me to command? Please, give me something truly hard and I will show you that even that will be observed to the letter by my faithful servants.”
Rabbi Yehoshua shook his head and said, “No, I believe that this is quite enough for you to do. After all, I would not like you to decree things which are too difficult for your people to obey.”
“Very well,” said the Roman. “If you insist, I will command my people to observe this ridiculously simple thing.”
And calling in his secretary, he dictated to him a royal proclamation forbidding all the people of the city to light fires for three days.
“Have copies of the proclamation made up,” he said, “and have it announced in all the marketplaces of the city.”
The emperor’s servants made haste to do his bidding and within a matter of hours the entire city was made aware of the emperor’s decree.
“The deed has been done, Rabbi Yehoshua,” the Roman said, “and in three days you will have to admit that I was right and am greater than Moshe your teacher.”
“We shall see what we shall see,” was all Rabbi Yehoshua answered.
That evening, the Roman emperor and Rabbi Yehoshua went up to the roof of the palace from where they could observe the entire city stretched out before them.
As the emperor looked out toward the horizon his face suddenly darkened. There, in the western part of the city, he could see smoke rising from a chimney.
“What is the meaning of this?” he muttered. Calling to his minister, he said, “Send soldiers to that house this very moment and find out who has dared to disobey the order of the emperor and light a fire.”
Within moments, horsemen had ridden off in the direction of the smoke. When they returned to the palace some time later, they went immediately to the emperor.
“Well, what was the matter? Who dared disobey my orders and lit the fire?”
“O, great Caesar,” said the soldiers, “we came to the house from where the smoke was rising and we made inquiries. It seems that one of the residents of the house had a slight illness and he called for a doctor.
“The physician assured him that it was nothing serious but advised him to drink hot broth. The man lit the fire in order to follow the doctor’s orders.”
When Rabbi Yehoshua heard this he turned to the emperor and said, “Behold now the difference between you and the great Moshe, our teacher. You gave orders that no one light a fire and, the very same day, while you were alive, one of your servants violated your commandment because of a trifling matter.
“Moshe, our teacher, on the other hand, commanded us, ‘You shall not light fires in all your dwelling places on the Sabbath day.’
‘All the days that the great Moses lived no Jew violated this commandment and it still stands after all the many years since he passed on.
“Let me ask you now: Who is really greater? You, who decreed a thing and whose decree was violated the very same day or Moshe, the son of Amram, who has been gone from this earth all these years and whose law still stands?”
“You are right, Rabbi Yehoshua,” admitted the Roman Caesar. “I made a great error in thinking that I was greater than your prophet Moshe, merely because he was dead and I am alive.”
Daf Drush 