The Fifth of November,
Gunpowder treason and plot;
For I see no reason
Why Gunpowder Treason
Should ever be forgot.
The greatest attempted act of terrorism in British history wasn't the work of jihadis but Roman Catholic British conspirators, the most well-remembered of whom is Guy (Guido) Fawkes, who were radicalized (one might say) by extremist Jesuit writings justifying the destruction of Protestant governments. In the conspirators' letters, Protestants and "atheists" are synonymous.
Their plot was to blow up the House of Lords on November 5, 1605 ("5/11" in modern UK dating) during the State Opening of Parliament when King James I would be present. This would, they thought, trigger a Roman Catholic uprising in England.
They nearly succeeded: successfully positioning 36 large barrels of gunpowder underneath the House of Lords—enough to blow the place to smithereens—which weren't discovered by authorities until the night before, the 4th of November.
The conspirators are easily romanticized. They were young—in their 30s—dashing, and determined. Most of them were undeniably brave, too, and committed to one another.
But make no mistake: they were ruthlessly cold-hearted. Their plan would have killed many, including innocents (assistants, pages, attendants, passersby, etc.). They were fully aware that the explosion would kill Roman Catholic lords, too. This did disturb some of the conspirators, enough for one to send warnings by anonymous letter. But it was not a great concern for most of the conspirators, and certainly not for the ringleaders.
Anti-Scottish prejudice was an additional motivation. Many of the conspirators were from families who'd become increasingly disconnected from political privilege largely as a result of prejudice against and suspicion of Roman Catholics. At the same time, Scottish gentlemen were rising in political influence. The conspirators hated the Edinburgh-born and very Protestant king himself, and they saw his desired program for a political union joining England and Scotland as one that would result in spiritual and political contamination.
Images: (top) Promotional still from the 2014 BBC documentary, Gunpowder 5/11: The Greatest Terror Plot; (bottom) The Gunpowder Plot Conspirators engraving (1605) of eight of the thirteen conspirators, by Crispijn van de Passe.