<span>Because Athens assumed an ever-more oppressive role, demanding tribute from league members.</span>
One theory suggested that a nomadic, Indo European tribe, called the Aryans, invaded and conquered the Indus Valley Civilization. The eastward shift of monsoons may have reduced the water supply, forcing the Harappans of the Indus River Valley to migrate and establish smaller villages and isolated farms.
Chang'an means of "<em>Perpetual Peace</em>" in traditional Chinese; and it can be described as an "<em>Everlasting Capital</em>" because it was persistently used as a capital again and again by many fresh Chinese rulers (<em>more than 10 dynasties</em>);
it can also be described as a <em>flourishing</em> cultural & <em>commercial center</em>; Chang'an has been one of the most important cities of the world.