Paris, 18th of August 1572. Numerous noblemen gathered in the Louvre to celebrate the marriage of the King Charles IX's sister Marguerite and the Protestant Henry III of Navarre. Many of the wedding guests had mixed feelings about this arrangement. True, a peace treaty had been signed two years before, which had put an end to the third war of religion between Catholics and Protestants. That paper peace, however, hadn't changed the deeply established malicious feelings of conservative Catholics against the Huguenots. The Queen Mother, Catherine de' Medici, had planned the marriage in order to consolidate the Huguenot friendly politics that found its origins with the peace in 1570. At the same time, with introducing Henry into her family, she would be able to exert her influence on him and on the higher officials of the Huguenot movement, and created a union that supported the crown. Many Protestants followed Calvin, who had declared that royalty misused the so-called Divine power, and therefore many Protestants were a direct opponent to the king. One of the wedding guests was Gaspard de Coligny was also invited to the wedding. The Admiral of France had become the military and political leader and had grown to be a danger for the crown. The Duke of Guise, one of the strongest opponents of the Huguenots, had a personal aversion against Coligny since he believed him to have ordered the assassination of his father Francis.
In spite of the tense atmosphere among the different religious fractions, the wedding festivities were celebrated grandly and lasted three days. But only a day after the ending of the celebrations, on August 22, De Coligny was shot at with an arquebus by a citizen named Maurevert. The attempted assassination failed, however, as the Huguenot leader was only wounded at the elbow and lost a finger, and those injuries were way too marginal to actually kill him. Maurevert escaped and it ever remained unclear whether he was hired by someone - and if so, by whom - or if he acted at his own initiative. Catherine de' Medici could have ordered his death because of his growing power over Protestant Parisians, but most likely Maurevert was hired by the Guise family, as the shot was fired from a house that was owned by the Guises.
The tense mood that already ruled in Paris at that time - most Parisians were fiercely anti-Huguenot and were highly offended by the presence of so many noble Protestants - was even increased by this attempted assassination. The Huguenots already had to cope with the hostile atmosphere in the capital and now they were confronted by attempted murder of their political leader. In the Louvre, the fear for a Huguenot revenge grew larger, and to ease moods the king hurried to visit Coligny on his sickbed the day after the attack. Charles emphasized once again that he desired a healthy relationship between the two religious branches, and promised the culprits - whoever they were - would be punished. However, that same day the queen mother was unpleasantly surprised by fanatic protestants who demanded justice when they burst in while she was having dinner. Coincidentally, a 4,000 man strong army commanded by Coligny's brother-in-law was camping just outside the Paris walls, which enhanced the fear for a Huguenot reprisal. In the same night, Catherine summoned her advisers in order to discuss how to handle the crisis. Because of the king's financial difficulties, but also because Catherine feared an alliance between Coligny, England and the Dutch Protestants who were in the middle of rebelling against Spain, a fourth war on religion was to be avoided. She advised the king to get rid of the Huguenot leaders. In his rage, the king declared all Huguenots should be destroyed, so there would be no chance of vengeance from that camp.
Directly after this decision, in the night of August 23 to 24 (the eve of the feast of Bartholomew), the city gates were closed by the officials and the Swiss Guard started to kill a list of Protestant officials. The Protestant noblemen that had come to the wedding or Marguerite and Henry III and who were still lodging in the Louvre palace were expelled and murdered outside without mercy. Together with a couple of his men, Henry, Duke of Guise himself rode to Coligny's house, dragged him from his sickbed, killed him and threw his body out of the window. Then the body was abused, castrated, hanged and burned by the raging Parision crowd, which had gotten wind of the killings in the mean time. Triggered by Coligny's assassination and stirred up by Henry de Guise, they began to hunt common Huguenots throughout the city. Streets were blocked in order to prevent Protestants from escaping, and thousands of people, including women and children, were brutally slaughtered. Their houses were burned in order to purify the city from heresy. A gruesome massacre with such dimensions was not intended by Charles IX and Catherine, but the furious people of Paris were unstoppable.
The massacre lasted for three days in spite of the official's attempt to stop the mob from further killings. Also the king's order to other cities in the province, which he had sent out on the 24th, could not prevent the people there from massacring Protestant inhabitants. Large communities of Huguenots were washed out, and even a larger number quickly converted to Catholicism to safe themselves from death. On top of that, many Protestants fled to safer countries. Although the number of French Huguenots was highly decreased, both religious parties prepared for a fourth civil war that would begin in the fall of 1572, and last until May 1573.
Book recommendation: "La Reine Margot" by Alexandre Dumas is a great historical novel on the events that initiated and followed the St Bartholomew's Day Massacre.