Original:  In 215 Caracalla travelled to Alexandria and responded to this insult by slaughtering the deputation of leading citizens who had unsuspectingly assembled before the city to greet his arrival  , before setting his troops against Alexandria for several days of looting and plunder.
Revised:  In 215 Caracalla travelled to Alexandria and responded to this insult by slaughtering the deputation of leading citizens who had unsuspectingly assembled before the city to greet his arrival in December 215 , before setting his troops against Alexandria for several days of looting and plunder.
Edit intent: meaning-changed