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Did Britain or Rome know the moments when Pax Britannia or Pax Romana had hit their tipping point to decline? I doubt it.
I think the tipping point will only be observable through the lens of history many years from now with a subject heading of: This event was the beginning of the end of Pax Americana.
The exact point will be the advent of the internet.
USA was still very much on the rise at the advent of the internet. If you define the advent of the internet to be Arpanet, then that was 1969, the same year the USA landed on the moon. If you define the advent of the internet to be the first use of the World Wide Web that would be 1989 the same year the Berlin wall came down and 3 years before the Soviet Union collapsed, which was arguably the most powerful the USA has ever been as it was before China's rise.