anzus age of discovery allies american century ancient egypt asian century athens axis powers bric balance of power balance of power in international relations black sea british empire byzantine empire carthage central powers china chinese century cold war correlates of war council on foreign relations digital object identifier egypt energy superpower europe france french colonial empire geopolitics great britain great power hans morgenthau hard power hegemony historical powers hyperpower israel japan korea mao zedong middle power military power projection mongolia nato napoleonic wars next eleven ottoman empire persian empire political power political realism potential superpowers power in international relations power politics power transition theory punic wars realpolitik regional power richard nixon roman empire roman republic russia russo-japanese war sassanid empire second superpower shanghai cooperation organisation sino-soviet split soft power soviet union sparta superpower superpower collapse superpower disengagement united states warring states period world war i world war ii