B • ▶ borders and frontiers introduction In the ancient world the borders and frontiers of kingdoms and states were most often delineated by natural features such as rivers and mountain ranges Many of these borders continued to be used in medieval times throughout much of the world, with Roman boundaries being retained in much of Europe, North Africa, and the Middle East In parts of China provincial borders were maintained from the Han Dynasty (202 b.c.e.–220 c.e.) To some extent the main changes took place in areas where old civilizations had collapsed and totally new entities emerged during the period of formation of medieval states The rise in the population of the medieval world, as well as the decrease in nomadic tribes, produced more areas under cultivation; as a result, borders had to be formed and agreed upon The delineation of boundaries in Europe, northern Africa, the Middle East, and large parts of mainland Asia grew in importance and were often defined by written treaties, drawn up mostly after conflicts, invasions, or incursions Because the treaties set down exact borders, it became more necessary to have a way of defining these boundaries, often by natural boundaries but also increasingly by markers, walls, or forts Rivers served as useful divisions between states The Danube, previously the boundary of the Roman Empire, became the northern border of the Byzantine Empire; the Indus River defined, for a period, the boundary between the areas ruled by the Umayyads and that remaining under Hindu rule; and the Havel River separated Saxony in the eighth century from the lands ruled by the Abodrites, a confederation of Slavic tribes Mountain ranges worked well in other parts of the world, such as between France and the kingdoms of northern Spain, between Spain and Portugal, along northern border of the kingdoms of Hindu India, and on the western extremes of the Tang Empire (608–907) of China There were also frontiers defined by man-made objects such as walls—Hadrian’s Wall and the Great Wall of China being the most famous of those surviving, both built in ancient times Watling Street, a Roman road, was to mark the boundary between the Saxon realms and the “Danelaw” (northern and eastern England, where the Danes held sway) during ninth and 10th centuries, and Offa’s Dyke delineated the frontier of England and Wales In other parts of the world, where there were no formally defined borders, many nomadic peoples in Africa, the Americas, and Australia tended to restrict themselves to particular parts of their respective continents, there being parts of those lands that were uninhabitable With small populations, relative to the land area, and people moving with the seasons, these tribes rarely clashed, except in some parts of Africa such as the Sahara, where new kingdoms emerged and their rulers wanted to expand the land under their control In the Americas the great empires of the Inca and the Maya and then the Aztec in Central America seem to have been 127