“It is well known that each country has a national anthem, which is sung on special occasions in the country ... The song represents the style of the land, and in this case it is especially clear; as it is well known that the song was composed during the revolution.
“A few years ago, Chassidim began to sing the French national anthem to the words of ‘Ho’aderes V’haemunah,’ and a short time later, a strange thing happened - the French government changed the notes of the niggun, altering its beat and softening its tone.
“Chassidim explain, that once the niggun was brought to kedusha, the sar and mazel - protective angel - of France felt it, and that brought about the change.” (Shabbos Parshas Vayeshev 5752)
The Rebbe was referring to what took place during hakafos on Simchas Torah 5734:
Ten years had passed since the Rebbe had taught a niggun on Simchas Torah; the last time was in 5724. The Rebbe explained that he stopped this custom along with a number of other things because Chassidim had not appropriately appreciated them, 33 so what took place during that hakafos came as a total surprise.
On the night of Simchas Torah, the Rebbe called the group of French visitors for a hakafah, as he had done the previous two years as well. Waiting for all of them to gather at the bimah, the Rebbe commented - in French - to one of the guests that, “I called all of the Frenchmen; did they all come? I see one there, and another there… Someone should go to the middle of the shul, and announce that all of the guests from France should come to the middle.”
Then, unexpectedly, the Rebbe turned around to the crowd with his siddur in his hand, walked to the edge of the bimah, and began to sing the words of ‘Ho’aderes Vehaemunah’ to the tune of the Marseillaise, the French national anthem. Excitedly, the French guests joined in with the Rebbe, and later that night, they taught the new niggun to the rest of the Chassidim in 770.
On Shabbos Bereishis the Rebbe explained that although ‘converting’ niggunim is not a capability within the spiritual reach of our generation, nevertheless, since the Alter Rebbe ‘opened the faucet’ and ‘converted’ Napoleon’s march, it has become possible for us to do the same.
After the sicha, the Rebbe once again called the entire group to come onto the bimah, and went on to do something unprecedented: He said a short sicha entirely in French, telling the Jews of France to make a revolution against the yetzer hara “as soon as possible”!
From then on, singing this niggun became quite common. When the guests from france would come to the Rebbe for a Yom Tov, the Rebbe would ask them to sing ‘their niggun.