Complex sentence: When on 13 November 1887 ("Bloody Sunday"), a Trafalgar Square rally addressed by Besant was violently broken up by the authorities, Shaw quickly left the scene; the events of the day convinced him of the folly of attempting to challenge police power.
Split into simpler sentences: When on 13 November 1887 ("Bloody Sunday"), a Trafalgar Square rally addressed by Besant was violently broken up by the authorities, Shaw quickly left the scene. The events of the day convinced him of the folly of attempting to challenge police power.