The costs of the Habsburg-Valois wars forced the French to increase taxes and borrow heavily. King Francis I (r. 1515–1547) also tried two new devices to raise revenue: the sale of public offices and a treaty with the papacy. The former proved to be only a temporary source of money: once a man bought an office, he and his heirs were exempt from taxation. But the latter, known as the Concordat of Bologna (see "France" in Chapter 12), gave the French crown the right to appoint all French bishops and abbots, ensuring a rich supplement of money and offices. Because French rulers possessed control over appointments and had a vested financial interest in Catholicism, they had no need to revolt against Rome.

Significant numbers of those ruled, however, were attracted to the Reformed religion of Calvinism. Initially, most French Calvinists, called Huguenots, lived in major cities, such as Paris, Lyon, and Rouen. By the time King Henry II (r. 1547–1559) died in 1559, perhaps one-tenth of the population had become Calvinist.

The feebleness of the French monarchy was the seed from which the weeds of civil violence sprang. The three weak sons of Henry II who occupied the throne could not provide the necessary leadership, and they were often dominated by their mother, Catherine de’ Medici. The French nobility took advantage of this monarchical weakness. Just as German princes in the Holy Roman Empire had adopted Lutheranism as a way to oppose Emperor Charles V, so French nobles frequently adopted Protestantism as a religious cloak for their independence. Armed clashes between Catholic royalist lords and Calvinist antimonarchical lords occurred in many parts of France.

Calvinist teachings called the power of sacred images into question, and mobs in many cities took down and smashed statues, stained-glass windows, and paintings, viewing this as a way to purify the church. Though it was often inspired by fiery Protestant sermons, this iconoclasm, or destruction of religious images, is an example of ordinary men and women carrying out the Reformation themselves. Catholic mobs responded by defending images, and crowds on both sides killed their opponents, often in gruesome ways.

A savage Catholic attack on Calvinists in Paris on Saint Bartholomew’s Day, August 24, 1572, followed the usual pattern. The occasion was the marriage ceremony of the king’s sister Margaret of Valois to the Protestant Henry of Navarre, which was intended to help reconcile Catholics and Huguenots. Instead, Huguenot wedding guests in Paris were massacred, and other Protestants were slaughtered by mobs. Religious violence spread to the provinces, where thousands were killed. This Saint Bartholomew’s Day massacre led to a civil war that dragged on for fifteen years.

What ultimately saved France was a small group of moderates of both faiths, called politiques, who believed that only the restoration of strong monarchy could reverse the trend toward collapse. The politiques also favored accepting the Huguenots as an officially recognized and organized group. The death of Catherine de’ Medici, followed by the assassination of King Henry III, paved the way for the accession of Henry of Navarre, a politique who became Henry IV (r. 1589 1610).

Henry’s willingness to sacrifice religious principles to political necessity saved France. He converted to Catholicism but also issued the Edict of Nantes in 1598, which granted liberty of conscience and liberty of public worship to Huguenots in 150 fortified towns. The reign of Henry IV and the Edict of Nantes prepared the way for French absolutism in the seventeenth century by helping restore internal peace in France.