Complex sentence: During the administration of Sir Robert Peel, Lord Palmerston led a retired life, but he attacked with characteristic bitterness the Webster-Ashburton Treaty with the United States, which closed successfully some other questions he had long kept open.
Split into simpler sentences: During the administration of Sir Robert Peel, Lord Palmerston led a retired life, but he attacked with characteristic bitterness the Webster-Ashburton Treaty with the United States. Much as he critized the Treaty, the Webster-Ashburton Treaty did, however, successfully close some other questions with which Palmerston had long kept open.