When the An Lushan Rebellion ended in 763, the Tang Empire had once again lost control over its western lands, as the Tibetan Empire largely cut off China's direct access to the Silk Road. ... These lands contained crucial grazing areas and pastures for raising horses that the Tang dynasty desperately needed.
Answer: The Bill of Rights consists of guarantees of civil liberties and checks on state power; it was added in order to convince states to ratify the Constitution.
