Pax Romana (Latin for "Roman peace") was the long period of relative peace and minimal expansion by military force experienced by the Roman Empire in the 1st and 2nd centuries AD. Since it was established by Augustus, it is sometimes called Pax Augusta. Its span was approximately 206 years (27 BC to 180 AD). The Pax Romana is said to be a "miracle" because prior to it there had never been peace for that many centuries in a given area of human history. 
Beginnings of relative peace
The Pax Romana started after Octavian (Augustus) met and defeated Mark Antony in the Battle of Actium on 2 September 31 BC. He became princeps, or "first citizen". Lacking a good precedent of successful one-man rule, Augustus created a junta of the greatest military magnates and stood as the front man. By binding together these leading magnates in a coalition, he eliminated the prospect of civil war. The Pax Romana was not immediate, despite the end of the civil wars, because fighting continued in Hispania and in the Alps. Nevertheless, Augustus closed the Gates of Janus (the Roman ceremony to mark world Peace) three times, first in 29 BC and again in 25 BC. The third closure is undocumented, but Inez Scott Ryberg (1949) and Gaius Stern (2006) have persuasively dated the third closure to 13 BC with the Ara Pacis ceremony. At the time of the Ludi Saeculares in 17 BC the concept of Peace was publicized, and in 13 BC was proclaimed when Augustus and Agrippa jointly returned from pacifying the provinces. The Ara Pacis ceremony was no doubt part of this announcement.
Augustus faced a problem making peace an acceptable mode of life for the Romans, who had been at war with one power or another continuously for 200 years. Romans regarded peace not as an absence of war, but the rare situation that existed when all opponents had been beaten down and lost the ability to resist. Augustus' challenge was to persuade Romans that the prosperity they could achieve in the absence of warfare was better for the Empire than the potential wealth and honor acquired when fighting a risky war. Augustus succeeded by means of skillful propaganda. Subsequent emperors followed his lead, sometimes producing lavish ceremonies to close the Gates of Janus, issuing coins with Pax on the reverse, and patronizing literature extolling the benefits of the Pax Romana.
Given the prominence of the concept of the Pax Romana, historians have coined variants of the term to describe systems of relative peace that have been established, attempted or argued to have existed. Such times have been credited to the British Empire during the 19th century. Some variants include:
- Pax Britannica
- Pax Americana
- Pax Hispanica
- Pax Mongolica
- Pax Ottomana
- Pax Sinica
- Pax Khazarica
- Pax Syriana
- Isaac Asimov's fictional Galactic Empire and Foundation series refer to Pax Trantorica and Pax Imperium.
- Pax Soprana is the sixth episode of the HBO original series The Sopranos.
- In Magi: The Labyrinth of Magic, Reim's Peace is the Reim Empire's version of Pax Romana, established about 200 years prior to the series by Empress Scherazade. Reim is a nation based on the Roman Empire.
- "Pax Romana". Britannica Online Encyclopedia., http://mathforum.org/library/drmath/view/64396.html
- "Augustus Caesar and His Role in the Pax Romana". Caltech.
- Augustus states in Res Gestae 13 that he closed the Gates three times, a fact documented by many other historians (See Gates of Janus).
- Scott Ryberg, Inez (1949). "The Procession of the Ara Pacis". Memoirs of the American Academy in Rome 19: 79–101.
- Stern, Gaius (2006). Women, children, and senators on the Ara Pacis Augustae: A study of Augustus' vision of a new world order in 13 BC.. ProQuest. ISBN 978-0-549-83411-3.
- Sir Ronald Syme had suggested a later date (but Rome was then at war).
- Momigliano, Arnaldo (1942). "The Peace of the Ara Pacis". Journal of the Warburg and Courtauld Institutes 5: 228–231.