Complex sentence: Both Manchus and Han Chinese joined the Han banners before Qing forces crossed over Shanhai Pass in 1644 and as such were both distinguished from Han Chinese who joined after 1644.
Split into simpler sentences: Both Manchus and Han Chinese joined the Han banners before Qing forces passed through Shanhai Pass in 1644. As such, they were both distinguished from Han Chinese who joined the Qing Empire after 1644.