Complex sentence: The war degenerated into a stalemate, with Matilda controlling much of the south-west of England, Stephen the south-east and the Midlands, with large parts of the rest of the country in the hands of local barons.
Split into simpler sentences: The war degenerated into a stalemate, with Matilda controlling much of the south-west of England, Stephen the south-east and the Midlands. Large parts of the rest of the country were in the hands of local barons.