Philippe-Emmanuel de Lorraine, Duke of Mercœur and of Penthièvre was a French soldier, a prince of the Holy Roman Empire and a prominent member of the Catholic League, who fought for Breton political independence from the House of Bourbon.

The Catholic League of France, sometimes referred to by contemporary Catholics as the Holy League, was a major participant in the French Wars of Religion. The League, founded and led by Henry I, Duke of Guise, intended the eradication of Protestantism from Catholic France, as well as the replacement of the French King Henry III, who had acquiesced to Protestant worship in the Edict of Beaulieu (1576). The League also fought against Henry of Navarre, the Protestant prince who became presumptive heir to the French throne in 1584.