The Fourth War of Religion (1572-73) was triggered by the St. Bartholomew's Eve Massacre of 24 August 1572, in which several thousand Huguenots were killed. The resulting war was dominated by a unsuccessful Catholic siege of La Rochelle, and was ended by a negotiated peace in the summer of 1573.
The massacre was an indirect consequence of the marriage of the Protestant leader Henry of Navarre and Marguerite of Valois, the sister of Charles IX. This marriage came at the end of a period of intensive marital negotiations (including the famous series of negotiations between Elizabeth I of England and the French court). The marriage was to be held in Paris, and so in August 1572 many of the main Huguenot leaders and their supporters arrived in the French capital. Amongst the guests were the young Prince of Condé and Admiral Coligny.
The young couple were betrothed on 17 August and married on the following day. The Huguenot leaders then remained in Paris for several days of celebrations, at the end of which Coligny expected to meet with Charles IX to discuss some breaches of the Peace of St. Germain. Catherine de Medici and Henry of Anjou were both worried about the influence Coligny was beginning to have over the king, and decided to assassinate him. Charles IX was not involved in this plan, and at this stage there appears to have been no plan for a wider massacre.
The assassination attempt was made on 22 August, and failed. Coligny was wounded but survived. Charles was greatly angered by the attempt and promised to find the culprits. It was only at this point that the plotters decided to carry out a wider massacre. The leaders in this new plot were once again Henry of Anjou and his mother, but this time they decided to win Charles IX over to their side. According to Henry of Anjou's own account of the plot Charles was told that the Huguenots were raising an army to take revenge for the attack on Coligny. The original plan was probably to murder Coligny and a handful of other Huguenot leaders.
Henry, duke of Guise was entrusted with the execution of the plot. Early on the morning of 24 August Guise led his men to Coligny's house where the Admiral was murdered. At about the same time Catherine de Medici ordered the tocsin of the church of St. Germain l'Auxerrois to be rung, and this triggered a wider massacre, led by the Royal garrison of Paris but that soon involved a large part of the Paris mob. Over the next few days several thousand people were killed in Paris. Condé and Navarre were spared, although both were forced to renounce their Protestant faith and became Catholics in October 1572.
The massacre quickly spread outside Paris, encouraged by a Royal letter to the provinces which made it clear that this was the king's will. Massacres began in Meaux and Troyes on 25 August, La Charité on 26 August, Orleans and Bourges on 27 August, Caen on 28 August and Lyons on 30 August, as well as at Rouen, Bordeaux and Toulouse. Far more people were killed outside Paris than in the capital.
A small number of cities held out against the massacre, and were retained by the Huguenots. Most important was La Rochelle, the main Huguenot base in many of the wars, but Sancerre on the Loire, Montauban, Nismes, Milhau, Aubenas and Privas were also held by them.
The main military action of the war was the Royal siege of La Rochelle, which began in December 1572 and lasted into July 1573. From February 1573 the siege was conducted by Charles IX's brother, Henry duke of Anjou, but despite being massively outnumbered the defenders managed to repel every attack on the city. The same happened at Sancerre, which was besieged from 3 January-19 August 1573.
By the summer of 1573 the court was looking for a way to end the war without losing too much face. Just at the right time news arrived that Henry of Anjou had been elected King of Poland. This allowed Charles to negotiate a peace treaty, and in early July he issued the Edict of Pacification at Boulogne. This actually reduced the amount of religious toleration granted to the Huguenots and restricted them to La Rochelle, Montauban and Nismes, where they were given the rights to free public worship.
This treaty failed to mention Sancerre, which was still holding out against Royal attack. The town held out until it was offered generous terms, although these don't appear to have been obeyed. Once again the peace that followed was uneasy and short-lived, and the Fifth War of Religion broke out in 1575.
The Fourth War of Religion is generally seen as the point at which a moderate Catholic faction, the Politiques, began to appear. This group, which included Montmorency, Biron, Cossé and Brissac, feared the Spanish more than the Protestants and supported the introduction of a limited amount of toleration in order to create a united France. The Politiques would begin to play a major role in events during the Fifth War of Religion.