Henri de Bourbon's marriage to Marguerite de Valois
On August 18, 1572, Henry IV — then Henri de Bourbon —, baptised as Catholic but raised in the Protestant faith, married Marguerite de Valois, a Catholic princess, in an arrangement to end the conflicts between Catholics and Huguenots in France. Their marriage took place on the parvis of the Notre-Dame Cathedral.
The wedding drew a great amount of huguenots to Paris and, in the night of August 23, a religious massacre began: the St. Bartholomew's Day Massacre.
On the video below, History professor Barbara Diefendorf explains the wedding and the massacre: