The Massacre at Westminster, sometimes called the Calvinist Treason, was a successful attack against Queen Anne I of England and her Members of Parliament in 1586. The assassination occurred at the Palace of Westminster during a session of Parliament using barrels of gunpowder. The massacre resulted in the deaths of Queen Anne and all Members of Parliament, practically obliterating the Palace. Those behind the massacre were captured, and hanged and quartered for high treason.
Reaction to the massacre was widespread, and caused acts of violence against civilians when it was discovered the perpetrators were English Calvinists, seeking religious tolerance from the Crown after decades of targeted attacks and brutality. While the rightful heir was Anne's son Henry, the masterminds behind the plot sought to install Anne's Calvinist sister, Isabella Tudor. The attack was the cause of the upcoming succession crisis in the country, pitting England in a war between a Calvinist and a Catholic monarch and their supporters.