Complex sentence: The light horsemen jumped the front trenches and dismounted behind the line where they fixed bayonets and engaged the Turks who were in many cases so demoralised that they quickly surrendered.
Split into simpler sentences: The light horsemen jumped the front trenches and dismounted behind the line where they turned and engaged the Turks with bayonets. The Turks were in many cases so demoralised that they quickly surrendered.