Complex sentence: The allies attacked and defeated king Olaf in the western Baltic Sea when he was sailing home from an expedition, in the Battle of Svolder, fought in September of either 999 or 1000.
Split into simpler sentences: The allies attacked king Olaf in the western Baltic Sea when he was sailing home from an expedition. Olaf was defeated in the Battle of Svolder, fought in September of either 999 or 1000.