Roman empire takes over Britain.
Keeping an army is too expensive.
Roman army is hated anyway.
Roman army leaves to fight barbarians.
Britain becomes the world superpower.
Keeping an army in the colonies is expensive.
Colonials do not like the British army.
British army goes home after a war.
Colonies call themselves the USofA
USA becomes superpower
USA takes over..... afghanistan? The next superpower is Afghanistan? Germany? Iraq? Vietnam?