Complex sentence: Chateaubriand decides to write up these "memories" in Rome, at the close of 1803; nevertheless, he does not commence writing them until 1809, although the different events in which he is surrounded prevent him to advance in this project.
Split into simpler sentences: Chateaubriand decides to write up these "memories" in Rome, at the end of 1803. Nevertheless he will not begin to write them until 1809, although the different events in which he is surrounded prevent him to advance in this project.