Some excerpts from Stuart Carroll’s Martyrs and Murderers: The Guise Family and the Making of Europe (Oxford, 2009)

All empires eventually overreach themselves and self-destruct. The Franco-British empire had hardly been founded before the cracks began to appear caused by unrelenting royal fiscal pressure and the disruption of trade. In Rouen, the hub of the new empire, peasants in the surrounding region were fleeing their homes to escape the new impositions ordered in the wake of the constable’s [Montmorency] defeat. [Montmorency failed to relieve St. Quentin in 1557 and was captured by Imperial forces; Francois, duc de Guise, brilliantly took the city as well as Calais in 1558, setting the stage for a treaty and for pressing the interests of the Guise with King Henri II.] It was at sea that the French could not compete. They were no match for the combined Anglo-Spanish navy…. But it was religion that would end the dreams of a Franco-British empire, and it was at the heart of this project that the Reformation rebellion would strike first…. In France, traditional politics, based on the struggle among rival factions, was about to be overturned by a new politics shaped by conflicting religious ideologies.

and

The death of Mary Tudor in November had already removed one of the main stumbling blocks to peace, namely Calais. The Treaty of Cateau-Cambresis, signed on 2 April between France and England and on the following day between France and Spain, was one of the most controversial in European history. It established the legal and political framework of Western European affairs and marked the beginnings of nearly a century of Spanish preponderance on the continent. Italy was abandoned by the French, who kept Calais and the three bishoprics of Metz, Toul, and Verdun. the veterans of Italy in particular were furious at what they considered a dishonorable peace. According to them, the king had ceded lands that cost 40 million crowns and 100,000 lives to win. Guise [Francois] became a spokesman for their discontent. The princes too felt that they had been sold out. Neither the King of Navarre nor the Duke of Bouillion gained compensation for the loss of their lands in the Treaty. Guise had made it clear that peace was an affront to his honor, for which he got widespread sympathy at court. He became the focus for those dissatisfied with the partisan rule of the man they snobbishly referred to as the ‘little baron from the Ile de France’ [i.e. Montmorency].

And then Henry II decided to joust just one more time.

Many in the Protestant leadership rejoiced at Henry’s death — their prayers had been answered, divine justice had delivered them. But we should be wary of interpreting events through Protestant eyes alone. Often written a decade or more after the events they purport to describe, the purpose of their accounts was to scapegoat the Guise and heap the blame for France’s decent into civil war and chaos on their shoulders. Other sources, such as the dispatches of the Spanish ambassador, Chantonnay, shed a different light on affairs.

And as for Catherine de Medici:

She prevented Diane de Poitiers from entering the king’s bedchamber; she never saw her lover again. When the king was unconscious she would keep Montmorency, whom she also disliked, at bay. While others had ignored the indignities that she had been forced to endure, the Guise had always accorded Catherine the respect that she craved. They also shared many of the same views, placing high priority on conquest in Italy and a low priority on religious persecution. She offered her support to them in return for the humbliing and banishment of Diane de Poitiers.

And as for heresy:

In the autumn of 1559 four new laws were added to the judge’s armoury, ordering the demolishing of [Protestant] meeting houses and the prosecution of any landlord harbouring Protestants…. In the outlying provinces the heresy laws remained a dead letter, either ignored or unenforced. Only in Paris did the cycle of death sentences against heretic’s outlast Henry’s death and accelerate under his son. … a handful of executions was not systematic enough to stop a movement that was developing into by far the largest unofficial Protestant Church in Europe. There was another option available. In Spain, Italy, and the Low Countries the Inquisition had stepped up its bloody business. The Hapburgs had already shown what might be achieved in Northern Europe. In the Low Countries, with its much smaller population, there were 1,300 executions for heresy between 1523 and 1565. A further thousand would follow during the bloody repression of 1567-69. In early December 1559 the Spanish ambassador complained about the low number of French executions and their quality — there was little value in burning people of ‘simple and base’ condition. He made it known to the Guise in the strongest terms that he ‘was not happy with the manner in which they were proceeding’.