What was the main cause of the 8 civil wars that were fought in france between 1562 and 1598?

He Massacre of Vassy, as this became known, provoked open hostilities between the two religions. The Bourbons, led by the prince of Condé, and proclaiming that they were liberating the king and regent from ‘evil’ councillors, organised a kind of protectorate over the Protestant churches and began to seize and garrison strategic towns along the Loire. Although the Huguenots had begun to mobilise for war before Vassy[17], Condé used the massacre as evidence that the Edict had been broken lending further weight to his campaign, and as hostilities broke out, the Edict was in fact revoked, under pressure from the Guise faction. 

The major engagements of the war occurred at Rouen, Dreux and Orléans. At the Siege of Rouen (May-October 1562), the crown regained the city at the cost of Antoine de Navarre, who died of his wounds. The Battle of Dreux (December 1562), saw the capture of Condé by the Guises and Montmorency, the governor general, by the Bourbons. In February 1563, at the Siege of Orléans, Francis, Duke of Guise was shot and killed by the Huguenot Poltrot de Méré; the Guise considered this an assassination on the orders of the duke’s enemy, Admiral Coligny, as it was outside of direct combat. The popular unrest caused by the 'assassination', coupled with the fact that Orléans was holding out in the siege, led Catherine to mediate a truce and the Edict of Amboise (1563). 

he "Armed Peace" (1563–67) and the "second" war (1567–68) 

The Edict of Amboise was generally regarded as unsatisfactory by all concerned, the Guise faction being particularly opposed to what they saw as dangerous concessions to heretics. Nonetheless the crown looked to re-unite the two factions in its efforts to re-capture Le Havre which had been occupied by the English as part of the Treaty of Hampton Court between the Huguenot leaders and Elizabeth I. The English were successfully expelled that July and the next month Charles IX declared his legal majority, ending Catherine de' Medici’s regency. His mother continued to play a principal role in politics, however, and she joined her son on a Grand Tour of the kingdom between 1564 and 1566, designed to reinstate crown authority. 

Reports of iconoclasm in Flanders led Charles IX to lend support to the Catholics there, leading to fears among the French Huguenots of a Catholic re-mobilisation against them. Philip II of Spain’s reinforcement of the strategic corridor from Italy north along the Rhine added to these fears and political discontent grew. Protestant troops then made an unsuccessful attempt to capture and take control of King Charles IX in the Surprise of Meaux, a number of cities, such as La Rochelle, declared themselves for the Huguenot cause and Catholics, both laymen and clergy, were massacred the following day in Nîmes in what became known as the Michelade. 

This provoked the Second War, the main military engagement of which was the Battle of Saint-Denis where the crown’s commander-in-chief and lieutenant general, the seventy-four year old Anne de Montmorency died. The war was brief, ending in another truce, the Peace of Longjumeau (March 1568), which granted significant religious freedoms and privileges to Protestants. 
The "third" war (1568–70) 

In reaction to the Peace, Catholic confraternities and leagues sprang up across the country in defiance of the law throughout the summer of 1568. Huguenot leaders such as Condé and Coligny fled court in fear of their lives, many of their followers were murdered, and in September the Edict of Saint-Maur revoked the Huguenots' freedom to worship. In November William of Orange led an army into France in order to support his fellow Protestants, but the army being poorly paid, he accepted the crown’s offer of money and free passage to leave the country. 

Nevertheless the Huguenots gathered together a formidable army under the command of Condé, aided by forces from south-east France, led by Paul de Mouvans, and a contingent of fellow Protestant militias from Germany — including 14,000 mercenary reiters led by the Calvinist Duke of Zweibrücken.[18] After the Duke was killed in action, his troops remained under the employ of the Huguenots who had raised a loan from England against the security of the queen of Navarre’s crown jewels [19]. Much of the Huguenots' financing came from Queen Elizabeth of England, who was likely influenced in the matter by Sir Francis Walsingham.[18] The Catholics were commanded by the Duke d'Anjou (later King Henry III) and assisted by troops from Spain, the Papal States and the Grand Duchy of Tuscany.[20] 
The Protestant army laid siege to several cities in the Poitou and Saintonge regions (to protect La Rochelle), and then Angoulême and Cognac. At the Battle of Jarnac (16 March 1569), the prince of Condé was killed, forcing Admiral de Coligny to take command of the Protestant forces, nominally on behalf of Condé’s 15 year old son, Henry, and the sixteen-year old Henry