Apparently my latin is garbage and it should be pax americana, but I digress....
For the US at least I would say we've been at peace (on our own soil) since the end of the civil war (if im not forgetting anything after that), maybe the closest you could get to a break would be the bombing of pearl harbor (and subsequently WW2), but in general I view the world wars as Europe having a slap fight until America decides to step in and put everyone in the timeout corner. Its undeniable that when we tell people to think twice, they generally listen to us, hence the analagous power of pax romana or pax mongolica