I'm not making this up to make excuses for the US' behavior. Pax Britannica and Pax Romana weren't entirely peaceful either. Pax Romana had an unusual absence of civil war, but near-constant conflict at the edges of the empire. Pax Britannica refers to the absence of wars between great powers, but did have some brutal colonial conflict - the Opium wars, scramble for Africa, etc. Pax Americana has been very peaceful compared to WW1 and 2.