Henry, Duke of Guise, a staunch Catholic and head of one of the most powerful noble families in France led his personal army to massacre a Huguenot congregation while they were peacefully conducting worship in the village of Vassy. This act of provocation led to open warfare between the Huguenots and the Catholics. 

At the time France was divided into three religio-political factions. The Bourbon family, which was closely related to the Royal Family and therefore inextricably linked to the throne, aligned themselves with the Huguenots. The Guise family which was powerful though not Royal was allied with the militant counter-Reformation set in motion by Pope Paul III at the Council of Trent. The monarchy itself was moderately Catholic, meaning though they were aligned with the Catholic church they tended to shy away from counter-Reformatory activities. 

After the Affair of the Placards the Reformation had been outlawed in France yet despite its illegal status, thanks to the droves of Huguenot ministers creeping into the country from Switzerland, the Reformation spread like wildfire. The Guise family blamed the monarchy for turning a blind eye to the steadily increasing threat while the Bourbon family threw its considerable political weight and wealth behind the subversive movement. All these elements combined to create a tense stand-off between each of the parties, which eventually devolved into open warfare.