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Another classic case: the fall of the USSR. Gorbachev's Peristroika was not meant to collapse the country. It was meant to open the country up and grow its economy, much like what China did. Neither the leadership of the USSR nor apparently US or NATO intelligence thought the USSR was about to collapse.

I personally think the US is much closer to collapse and fragmentation than anyone would suspect. If a time traveler told me I'd be living the Pacific States of America (PSA) in 2035 and that this new nation would maintain a NATO-like alliance with Canada and Mexico, I would not be terribly shocked.

Things can be more fragile than they appear and definitely more fragile than their designers and operators think.

One more non-political case that comes to mind is the collapse of World Trade Center Building 7, a building that was not hit by a plane and is a frequent topic of controlled demolition conspiracy theories. A large fire broke out in the building apparently fueled by diesel fuel that was being stored on site. That should not have caused the building to collapse, but it did. Maybe the building wasn't as strong as people thought either because of a design flaw or corners cut during construction. I once read an analysis that ended "moral of the story? If there's a large fire in a modern multi-story office building, you might want to stand back."