On either 30 June or 1 July 1559, during a jousting match to celebrate the
Peace of Cateau Cambrésis between Henry II and his longtime
Habsburg enemies, a splinter of wood from Montgomery's shattered
lance pierced Henry's eye and entered his brain, mortally injuring him. From his deathbed Henry absolved Montgomery of any blame, but, finding himself disgraced, Montgomery retreated to his estates in
Normandy. There he studied
theology and converted to Protestantism, making him an enemy of the state.
The fatal tournament between
and Montgomery (Lord of Lorges).
In 1562, Montgomery allied himself with another Protestant convert,
Louis I de Bourbon, prince de Condé. He was one of the few refugees to survive the
St. Bartholomew's Day massacre after a wounded
Huguenot swam across the
Seine to warn him that rioting had begun. He took control of
Bourges and during September and October defended
Rouen from the Royal Army. A price was put on his head, but he managed to escape to
England. The queen mother,
Catherine de' Medici, asked
Queen Elizabeth I for his
, but Elizabeth refused.
Montgomery returned to France with a fleet in an attempt to relieve the
Siege of La Rochelle in 1573. The following year he attempted an insurrection in Normandy, but was captured, taken to Paris,
 and sentenced to death. On 26 June 1574, as he was about to be beheaded, Montgomery was informed that a royal
edict had proclaimed that his property would be confiscated and his
children deprived of their titles.
A freely adapted version of Montgomery's life is told in
Alexandre Dumas' novel
The Two Dianas.