The Pax Americana has never been free of contentiousness or outright war. People (especially young people) forget how fragile things were prior to the fall of the USSR. The complete US dominance of the '90s and '00s is not the norm. And just as the Soviet house of cards collapsed, there's no guarantee that China's ascendancy is indefinite. Russia's posturing right now is a veil for its domestic strife and decrepit military, they are not much of a force.