Complex sentence: In view of his probable accession to power, preparations were made in the City of London for a banquet and a general illumination to celebrate the event but the celebration was at once countermanded when it was known that he had become Earl of Chatham.
Split into simpler sentences: In view of his probable accession to power, preparations were made in the City of London for a banquet and a general illumination to celebrate the event. But the celebration was at once countermanded when it was known that he had become Earl of Chatham.