She is next found attempting the assassination of the Duke of Condé, but she failed to accomplish that crime because her son, the king, refused his consent. Soon after, Francis II. died, it is said from the effect of poison dropped into his ear while he was sleeping; it is probable that this crime was committed at the instigation of the mother, since by his death and the accession of Charles IX. she became regent (1560). She was then all-powerful and in a position to exercise her long dormant talents.
Her first plan was to incapacitate all her children by plunging them "into such licentious pleasure and voluptuous dissipation that they were speedily unfitted for mental activity or exertion." Most unprejudiced historians credit her with the Massacre of Saint Bartholomew; she is said to have boasted about it to Catholic governments and excused it to Protestant powers. For a number of years, she had been planning the destruction of the Huguenot princes, and as early as 1565 she and Charles IX. had an interview with the Duke of Alva (representative of Philip II), to consult as to the means of delivering France from heretics. It was decided that "this great blessing could not have accomplishment save by the deaths of all the leaders of the Huguenots."
That fearful crime, the bloody Massacre of Saint Bartholomew, is familiar to everyone. The only excuse offered for this most heinous of Catherine's many offences is her intense sentiment of national unity; the actual reason for it is to be sought in the fact that as long as the Protestants retained their prestige and influence, Catherine and her Catholic party could not do as they pleased, could not gain absolute control over the government. History holds her more responsible than it does her weak son. The climax came on the occasion of the wedding of Marguerite of Valois with the Prince of Navarre, which meant the union of the branches--the Catholic and the Protestant. This resulted in the first breach between the king and Catherine; the latter at that time perpetrated one of her dastardly deeds by poisoning the mother of the Prince of Navarre--Jeanne d'Albret, her bitter enemy.
After the death of Charles IX., Henry III. was the sole survivor of the four sons of Catherine. Although her power was limited during his reign, she managed to continue her murderous plans and accomplished the death of Henry of Guise and his brother the cardinal, which crime united the majority of the Catholics of France against the king and was the cause of his assassination in 1589. This ended the power of Catherine de' Medici; when she died, no one rejoiced, no one lamented. Wherever she had turned her eyes, she had seen nothing but occasions for uneasiness and sadness; she had retired from court, feeling her helplessness and disgrace as well as the decline in power of that son in whom her hopes were centred. She decided to reënter the scene of action and save Henry. The stormy scenes of the Barricades and the League and the murder of the Duke of Guise hastened her death, which occurred in 1589.