Complex sentence: Wellington House began its propaganda campaign on September 2,1914 when Charles Masterman invited twenty-five leading British authors to Wellington House, the headquarters of the War Propaganda Bureau, to discuss ways of best promoting Britain's interests during the war.
Split into simpler sentences: Wellington House began its propaganda campaign on September 2nd, 1914 when Charles Masterman invited twenty-five leading British authors to Wellington House. Once arriving at the headquarters of the War Propaganda Bureau, their agenda was to discuss ways of best promoting Britain's interests during the war.