He became notorious for his persecution of the Vaudois (Waldensians) culminating in the massacre of 1655, known as Piedmontese Easter. The massacre was so brutal that it prompted the English poet John Milton to write the sonnet "On the Late Massacre in Piedmont". Oliver Cromwell, Lord Protector, called for a general fast in England and proposed to send the British Navy if the massacre was not stopped while gathering funds for helping the Waldensians. Sir Samuel Morland was commissioned with that task. He later wrote "The History of the Evangelical Churches of the Valleys of Piemont" (1658). The 1655 massacre was only the beginning of a series of conflicts, the Savoyard–Waldensian wars (1655–1690), that saw Waldensian rebels use guerrilla warfare tactics against ducal military campaigns to enforce Roman Catholicism upon the entire population.