There's always some kind of monocausal influence claimed, but really that was like the Boer War in the early 1900s. America was certainly at top power in the last twenty years, but its alliance was already fracturing. The Western NATO members were pushing more of their productive capacity into social services and forming strong dependencies on Russian fossil fuels. China's ascendance also meant that the encirclement that US's presence in Europe + Taiwan + Japan (and those governments themselves) kept was going to need to be extra tight.

But Europe couldn't keep herself together, Taiwan was constrained by circumstances to not defence-spend-up and Japan is just moribund despite attempts to rebuild. Realistically, the US kept everyone together for some 40 years after the Berlin Wall fell and that's a pretty good run. Two generations in "Whitey's on the Moon" is a resurgent and wide culture, and China outproduces any other nation while domestically and internationally repudiating that culture.

Perhaps we were doomed to this path by the inexorable nature of success. Two generations have been born and enough time has passed that people have forgotten what it is like to fear the "awesome Soviet threat". The modern empire was a loose confederation of US-Europe and the East-Asian satrapies with a capital in DC perhaps but other capitals in London and Paris as well. And just like that Boer War showed the old British Empire could bleed so will Iran have done the same this time.

Doubtless when the need arises we will sweep away environmental law and historical protection law in order to build our factories but already the appetite for war is gone from America. Why Europe couldn't keep herself together and why America couldn't retain the alliance and why the modern Not-Empire fell will probably be written about, but I think it's worth remembering Kipling at the Diamond Jubilee of Queen Victoria who was then queen over an indomitable empire:

    Far-called, our navies melt away;
      On dune and headland sinks the fire:
    Lo, all our pomp of yesterday
      Is one with Nineveh and Tyre!
    Judge of the Nations, spare us yet,
    Lest we forget—lest we forget!

Or in the more elementary school warning manner: "This too shall pass". For my part, I certainly hope that "government of the people, by the people, for the people, shall not perish from the earth" and that means our mighty opponents should not prevail because that is not their way of life. And certainly I do not think that lashing out at our allies or attempting to take for ourselves land which is nonetheless in this larger Not Empire is the way to ensure that.

At best, I hope that the Iran War teaches us where we are weak and we are wise enough to learn this, and I hope that the Not-Empire heals and order is restored in this world.