Complex sentence: Originally intended as a relief force for the British garrison that was briefly besieged at Newport, Rhode Island, Grey's force of 4,000 arrived after the Americans had already retreated, and was diverted for raiding by British army officer General Sir Henry Clinton.
Split into simpler sentences: Originally intended as a relief force for the British garrison that was briefly besieged at Newport, Rhode Island, Grey's force of 4,000 arrived after the Americans had already retreated. Thus Grey's force was diverted, for raiding, by British army officer General Sir Henry Clinton.