Complex sentence: It was further weakened by economic factors such as the collapse of Rus' commercial ties to Byzantium due to the decline of Constantinople and the falling off of trade routes, it finally fell to the Mongol invasion of the 1230s.
Split into simpler sentences: It was further weakened by economic factors such as the collapse of Rus' commercial ties to Byzantium due to the decline of Constantinople and the accompanying diminution of trade routes through its territory. The state finally fell to the Mongol invasion of the 1230s.