Pax Americana is kind of riddiculous name for the fact that US has started over 200 military conflicts since WW2. America is at war 93% of it's existence:
http://www.washingtonsblog.com/2015/02/america-war-93-time-2...
http://www.washingtonsblog.com/2015/02/america-war-93-time-2...
I think, to be meaningful, Pax Americana must refer to an "empire" larger than the territory of the United States. There hasn't been a war in Western Europe since 1945; the longest lasting war in South America was the Falklands War at 74 days.