The Edict of Nantes was issued in 1598 by Henri IV of France to grant French Protestants (also known as Huguenots) equal rights with Catholics.
The Edict was introduced primarily to end the long-running, disruptive French Wars of Religion. Henri IV also had personal reasons for supporting the Edict. Until assuming the throne Henri himself had been a Protestant, and he remained sympathetic to their cause: he converted in order to become king, famously saying, "Paris is worth a Mass." The Edict succeeded in restoring peace and internal unity to France for many years.
In 1685, however, Louis XIV renounced the Edict and declared Protestantism illegal with the Edict of Fontainebleau. This had very damaging results. While the wars of religion did not reignite, many Protestants did choose to leave France, most moving to Great Britain, Germany and the Dutch Republic. This exodus deprived France of many of its most skilled and industrious individuals, who would from now on aid France's rivals. The revocation of the Edict of Nantes also further damaged the perception of Louis XIV abroad, making the Protestant nations surrounding France even more hostile.