Complex sentence: Himilco was able to advance unopposed along the north coast of the island, and took and destroyed Messana (modern Messina); from whence he advanced upon Syracuse, his fleet, under Mago, supporting the operations of the army.
Split into simpler sentences: Himilco was able to advance unopposed along the north coast of the island, and took and destroyed Messana (modern Messina); from where he advanced on Syracuse. Himilco's fleet, under Mago, supporting the operations of the army.