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Catholic League

Many leading Roman Catholics were appalled by Catherine's attempts to appease the Huguenots. After the Edict of Beaulieu, they had started forming local leagues to protect their religion. The death of the heir to the throne in 1584 prompted the Duke of Guise to assume the leadership of the Catholic League. He planned to block Henry of Navarre's succession and place Henry's Catholic uncle Cardinal Charles de Bourbon on the throne instead. In this cause, he recruited the great Catholic princes, nobles and prelates, signed the treaty of Joinville with Spain, and prepared to make war on the "heretics". By 1585, Henry III had no choice but to go to war against the League. As Catherine put it, "peace is carried on a stick" (bâton porte paix). "Take care", she wrote to the king, "especially about your person. There is so much treachery about that I die of fear."

Henry was unable to fight the Catholics and the Protestants at once, both of whom had stronger armies than his own. In the Treaty of Nemours, signed on 7 July 1585, he was forced to give in to all the League's demands, even that he pay its troops. He went into hiding to fast and pray, surrounded by a bodyguard known as " the Forty-five ", and left Catherine to sort out the mess. The monarchy had lost control of the country, and was in no position to assist England in the face of the coming Spanish attack. The Spanish ambassador told Philip II that the abscess was about to burst.

By 1587, the Catholic backlash against the Protestants had become a campaign across Europe. Elizabeth I of England 's execution of Mary, Queen of Scots, on 8 February 1587 outraged the Catholic world. Philip II of Spain prepared for an invasion of England. The League took control of much of northern France to secure French ports for his armada.