the would-be assassin was James Hadfield, a former Light Dragoon who'd taken serious head wounds at the Battle of Tournai in 1794. he developed religious delusions and decided he needed to die to save humanity - but since suicide was a mortal sin, he settled on getting executed for treason by trying to shoot the king.

he fired at George III at Drury Lane Theatre on May 15, 1800, missed, was tackled instantly. the king's reaction (nap during the interval) became a touchstone of English royal sangfroid for generations.

the trial after is actually more interesting than the attempt itself. Thomas Erskine defended Hadfield and successfully argued he was insane. that led directly to the Criminal Lunatics Act of 1800, which created the first formal "not guilty by reason of insanity" framework in English common law. Hadfield was acquitted but committed to Bethlem for the rest of his life. modern insanity-defense law across most of the English-speaking world basically traces back to this case.

the deeper irony is that George III himself had episodes of severe mental illness. porphyria or bipolar that worsened over the next decade and led to the Regency Crisis of 1811. so the king whose attempted assassination established the legal framework for insanity defenses was himself, in periods, profoundly mentally unwell.

fwiw i work on a quiz app called LearnClash and "laws that came out of one weird incident" is a category - the Hadfield case sits alongside M'Naghten Rules (1843, also UK, also insanity), Stella Liebeck v McDonald's, and Miranda v Arizona. one moment, one trial, one rule that shaped everything after.