The United States is a political anomaly. Throughout time there has never been a nation so politically, culturally, and militarily dominant. Rome, even at its height, had rivals. So too did the British Empire, which at its apex made pretense to the rule of the waves, in spite of near constant challenges to its power from forces seeking to upset or supplant it. The international stability and peace created by these great empires, the Pax Romana and Pax Britannica, the Roman Peace and the British Peace, served in their times to guarantee security and relative prosperity within their spheres of influence. Yet they could never do so unchallenged.