When the Protestant Reformation created an entirely new branch of Christianity in the mid-16th century, the Catholic Church smarted from the break for some time. In addition to losing millennia-old face, the church risked losing land, power and funding, as formerly all-Catholic areas began to turn towards Protestantism.
This was the atmosphere under which Paris found itself in August 1572, when the city was filled with both Catholics and visiting Huguenots, or French Protestants. These two warring groups were in town for the marriage of a Catholic noblewoman to a Huguenot aristocrat. Almost as soon as the wedding ended, the Catholic French king, Charles IX, decided that the Huguenot military leader may as well be captured and killed for his trespasses against the church. To ensure that he needn't hear any complaints from the visiting Huguenots, he ordered all of the ones found in Paris killed as well. Over the course of just a few days, in what came to be called the St. Bartholomew's Day Massacre, between 1,000 and 4,000 French Protestants were murdered in Paris.
For good measure, King Charles carried the massacre into the countryside, ordering the revenge killings against all Huguenots found in France, leading to the murders of between 30,000 and 100,000 of them following the initial massacre [source: Oberhofer].This is how the Huguenots came to live in England.