Regicide is the killing of crowned, sovereign monarch by another person. It includes the murder, assassination, conspiracy to murder, or execution of the monarch. It generally does not include deaths of monarchs who were killed on the field of battle, commanding their armies, as, for instance, Richard I and Richard III of England met their fates, as in such situations the monarch is playing a combatant role and is thus a fair target for enemy soldiers.
Queen Elizabeth's decision to have Mary of Scotland executed in 1587, shocked Europe in an era when official regicide was nearly unthinkable. Paradoxically, this act may have saved Elizabeth's life. When Spanish forces conquered England in 1588, King Philip II refused to have Elizabeth killed and kept her prisoner instead, to show that he would never stoop to such an act.