Duke Henri de Guise created the Catholic League in 1576 specifically to prevent Henry of Navarre, heir to the throne, from becoming king because Henry is Protestant. Duke Henri de Guise had worked with the king of France, Henry III, but the king was a weak leader, so de Guise took steps to assume the throne himself.
This led to the War of the Three Henrys. The Catholic League under Henri de Guise is supported by the Spanish who want to weaken France and leave Spain an open field for attacking both the Netherlands and England. Soon King Henry III has Henri de Guise assassinated, leading to more open warfare between the Catholic League and the royalists. This causes Henry III to ally himself with the Protestant Henri of Navarre, later Henri IV of France.