The so-called "War of the Three Henrys" was the eighth and last of a brutal series of wars in the second half of the 16th century known collectively as the "French Wars of Religion." The eponymous "three Henrys" were Henry, duc de Guise, the fanatical leader of the extremist Catholic League, Catholic French king Henry III, and Protestant heir to the French throne Henry of Navarre (later King Henry IV).
The war erupted in 1585 when Henry, Duke of Guise succeeded in pressuring King Henry III into signing the Treaty of Nemours, which revoked all previous concessions to the Huguenots and excluded the protestant Henry of Navarre, the rightful heir to the French throne, from being named Henry III's successor. Henry of Navarre immediately took up arms to defend his claim to the throne, drawing the eager duc de Guise and the reluctant Henry III into a three-way struggle for control of France.
Backed with financial aid of the English, who were always keen to help out French protestants, Henry of Navarre launched a campaign in Guienne in 1586 and notched an early victory at the Battle of Coutras in October of 1587, defeating and killing Henry III's favorite, Anne, duc de Joyeuse. Meanwhile, the Duke of Guise twice defeated an army of German Protestants, who had marched into France to join Navarre, at the battles of Vimory and Auneau. De Guise then marched triumphantly to Paris in defiance of a direct order from Henry III that he keep clear of the capital.
When Henry de Guise reached the city in early 1588, the jubilant pro-Catholic Parisians, incited by the presence of the army of the Catholic League, revolted against the King, who they viewed as too soft on Protestants, and expelled him from the capital in the so-called "Day of the Barricades." De Guise erred, however, in allowing Henry III to keep his life in exchange for political concessions to the League, and was cruelly rewarded for his clemency when the king arranged to have both him and his brother Louis assassinated later that same year. Meanwhile, the revolt led to rapproachment between King Henry and Henry of Navarre, leading the king to officially recognize Navarre as his heir.
But despite the elimination of one of the Henrys, the War of the Three Henrys was far from over, and in fact only further intensified over the course of 1589. Enraged by the assassination of their leader, the Catholic League went into open rebellion against King Henry, declaring him deposed and acclaiming the Cardinal de Bourbon as "King Charles X" in League-controlled Paris. The two remaining Henrys thereupon teamed up to beseige Paris, but the seige was broken when a fanatical Catholic monk assassinated Henry III and then his Catholic troops refused to support the protestant Henry of Navarre despite his proclaiming himself King Henry IV. Moreover, the Spanish now intervened on behalf of the League, seeing a golden opportunity to get rid of the troublesome Henry of Navarre once and for all.
Nevertheless, the last remaining Henry managed to renew his assault on Paris, winning a major victory over combined Spanish and League forces at the Battle of Arques, but without the aid of Henry III's old supporters, he was unable take Paris, and was forced to withdraw.
As soon as the Spanish withdrew, Henry IV renewed his assault in 1590, and again defeated the forces of the League at the Battle of Ivry, but his third attempt to capture Paris failed when the League defenders were once again reinforced by a Spanish army under Alexander Farnese, Duke of Parma, who was hastily recalled to France from Holland when it looked like Paris was about to fall.
In 1592, however, Parma was called back to Holland after a serious of disastrous defeats by Spanish forces at the hands of the feisty Dutch, once again leaving Henry free reign to attack Catholic strongholds. So Spanish king Philip II was yet again forced to order Parma back into France to relieve Henry's Siege of Rouen. Parma succeeded in breaking the siege, then suddenly died on his way back to Holland. The Spanish were thus deprived of their most brilliant general, but the wars in Holland and France dragged on.
In 1593, Henry finally realized that there was only one way to end the war, and thus renounceed Protestantism and accepted Catholicism. Although many Catholics saw this for the cynical ploy that it was, it abated the hatred in enough of them to finally allow Henry to enter Paris the following year and be officially crowned king of France. Having fought so hard for so many years over his Protestant faith, it was rather unexpected for Henry to suddenly renounce it all and become Catholic, but he was simply exhausted after three failed attempts to take Paris, and thus decided, as he himself famously explained, that "Paris is worth a Mass."