I cannot speak as detailed about Mongolia, Britannica, and Americana However I can speak to them in context to the Pax Romana which I will describe.

The major problem in comparing all of these separate "peaces" is the difference in time, societies, concepts of war and militarism, and overal frequench of war. And as well there is never really a time truly devoid of war and conflict.

The Pax Romana was not a time devoid of war, there were lots of them, the civil war of 68-69 and year of the four emperors, the bar kokhba revolt, the Parthian wars, Dacian wars, and the civil war ending the Nerva-Antonine dynasty. Simply put, the Pax Romana was a time of less frequent wars as before and after, the wars were almost continuous during the Roman Republic.

This, however was a symptom of many social and economic issues, chief amoung them the need for wealth and social prestige. The Roma societal elite required a steady stream of battles and honors to both feed the coffers of Rome and its patricians as well as supply a steady supply of militarily experienced noble youth who could proceed to gain office. During the Empire, these social pressures leased slightly as the emperor became consul and tribune for his lifetime. Military conquest for profit started to take a secondary role I Roman economics to the trade of luxuries and food, however were still extraordinarily important to the individual commanders and to the state itself.

In a sense, Augustus unified his conquered empire bringing increased stability and trade to Roman territories and provinces. The period itself ended when the stability of Rome became questionable due the the 3rd century crisis and a lack of stable central authority.

The Pax Romana was a time were there were no major powers capable of dealing serious reverses to the Roman Empire on the scale and dire nature as had Carthage, allowing the focus on trade and development of greater prosperity. This also engendered a more cohesive view of the empire as territories once conquered were incorporated into a more cohesive Roman social unit. In effect the Pax Romana allowed Rome the time to focus on social and economic growth instead of territorial growth.

With economic growth there developed the conservative element proposing traditional morals as the avenue to true greatness, a focus on the martial history of the Roman republic in contrast to the empire decrying the amassing of wealth and luxuries as decadence.

The end of this comes when that stability was shattered and no strong dynastic line was establish as had occurred with the Flavians, Nerva-Antonine, and Severans.

Despite the difficulty in comparing the time periods of empires, the common theme is the lack of an equivalent enemy capable and interested in competing with other power for dominance. All of these come from a time after an empire solidifies its hold after all major challengers have been defeated: after the Mongols defeat nearly all of Asia, the Romans defeat Carthage and Augustus defeats mark Antony providing stable, centralized political authority, after Britain and the coalition defeat napoleonic France, and after the defeat of Nazi Germany and the final death of European imperial power.

The demise of the peace usually stems from political instability or a stronger challenger state eventually usurping the hegemony of the primary state: the crisis of the third century in Rome, the death of the Mongol empire from lack of effective central authority, and the mutual suicide of European imperial powers in WWI and WWII.

I will not speculate about the end of the Pax Americana as it has yet to conclude, however the international political atmosphere has become significantly more complex with the end of the Cold War.

Coinage and Identity in the Roman Provinces edited by Christopher Howgego, Volker Heuchert and Andrew Burnett

Pliny the Elder: Natural Histories

Imperial Ideology and Provincial Loyalty in the Roman Empire By Clifford

I wrote this post (https://reddit.com/r/AskHistorians/comments/48vphv/modern_political_commentators_like_to_compare_the/) a while ago about comparing empires theoughout history. It is relevant only in the pitfall of comparing events so far distant in time from each other.

Typed on mobile so please excuse formatting or odd typos due to autocorrect. Let me know and I will correct them.